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References: <20220602213544.68273-1-max@enpas.org>
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From:   Vincent Mailhol <vincent.mailhol@gmail.com>
Date:   Sat, 4 Jun 2022 00:12:39 +0900
Message-ID: <CAMZ6RqLLn3tE6kPd+yyHURsbBU_jVKN=u4=YvhZVyAf_NyRqCQ@mail.gmail.com>
Subject: Re: [PATCH v7] can, tty: can327 CAN/ldisc driver for ELM327 based
 OBD-II adapters
To:     Max Staudt <max@enpas.org>
Cc:     Wolfgang Grandegger <wg@grandegger.com>,
        Marc Kleine-Budde <mkl@pengutronix.de>,
        linux-can@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Oliver Neukum <oneukum@suse.com>, linux-kernel@vger.kernel.org
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Precedence: bulk

Hi Max,

I gave a final deep look into your driver. I found a few things which
went through the cracks of my previous reviews. But overall, it start
to look good. Well done and looking forward for the v8.

On Fri. 3 June 2022 at 06:36, Max Staudt <max@enpas.org> wrote:
> This is the can327 driver. It does a surprisingly good job at turning
> ELM327 based OBD-II interfaces into cheap CAN interfaces for simple
> homebrew projects.
>
> Please see the included documentation for details and limitations:
> Documentation/networking/device_drivers/can/can327.rst
>
> Cc: linux-can <linux-can@vger.kernel.org>
> Signed-off-by: Max Staudt <max@enpas.org>
> ---
> Changes in v7:
>  - Accelerate parsing by not re-checking previous data
>      While waiting for a <CR> the code would search through previous
>      received characters, too. Keep track of what we've already
>      checked to speed this up.
>  - Speed up can327_is_valid_rx_char() with a LUT
>  - Make elm327_rxbuf_cmp() return bool
>  - Simplify/clarify branching and locking in several places
>  - Don't ____cacheline_aligned the TX buffer
>  - Style and comments cleanup
>
> Changes in v6:
>  - Renamed 'elmcan' to 'can327'

Are elm327 and can327 synonymous?

>  - No other changes.
>
> Changes in v5:
>  - Fixed lockdep asserts (I didn't have lockdep enabled in my build)
>  - Restore: TTY wakeup request before write() in elm327_send()
>  - Clear TTY wakeup on uart_side_failure
>  - checkpatch style fixes
>  - Repack struct elmcan
>
> Changes in v4:
>  - Rebased on top of v5.18-rc5
>  - Simplified TTY locking.
>      The ldisc layer already blocks our ldisc ops from running concurrently.
>      No elm_get(), no RCU are needed, except locking against the TX worker.
>  - Removed .hangup() ldisc op which only called .close().
>      The ldisc layer calls .close() in a good place anyway.
>  - Restart on netdev down/up, even after UART failure.
>      This is in line with Marc's line of thinking, and there was no
>      good reason not to implement it. It also cleaned up the code.
>  - hw_failure is now uart_side_failure and shutdown looks closer to es58x.
>  - Clarified memory/string comparisons.
>      One helper function remains, but it's hopefully clear now :)
>  - lockdep_assert_held() instead of comments.
>  - Cleaned up types in struct elmcan and used pahole to pack it.
>      unsigned instead of int, u8 instead of char, ...
>  - The TX buffer is now a static part of struct elmcan. No more kmalloc().
>  - Removed dummy mailbox_read() for rx_offload.
>      by using can_rx_offload_add_manual() instead of can_rx_offload_add_fifo().
>  - netif_wake_queue() is moved to happen as late as possible.
>      Wake when the ELM327 is going into monitoring mode, not every time
>      after flushing whatever UART buffer we were TX'ing.
>  - Use alloc_can_skb() and avoid late allocation and copy.
>  - elmcan_netdev_start_xmit(): Used can_dropped_invalid_skb().
>  - Replaced ->can_dlc with ->len.
>  - Clarified size of local TX buffer in elm327_handle_prompt().
>  - Renamed TODO_* to ELM327_TX_DO_* and ELM_* to ELM327_STATE_*.
>  - Minor string and other cleanups.
>      pr_fmt, MODULE_*, unnecessary checks, ...
>  - Clarified file header.
>
> Changes in v3:
>  - Now depends on c2faf737abfb for new ldisc number 30.
>  - Eliminated hardcoded string lengths (GCC will work its magic).
>  - Emit generic error frames if an error message couldn't be parsed.
>  - Silence driver startup and init (but still announce ldattach).
>  - Cleaned up comments, strings, readme.
>  - Removed sole module option "accept_flaky_uart".
>    I likely had EMI in earlier testing, which is gone now.
>    This means we can stay strict, unless anyone objects.
>
> Changes in v2:
>  - Moved to SocketCAN's rx-offload wrapper for NAPI, thus avoiding
>    packets being reordered.
>  - Updated TTY ldisc code for Linux v5.17-rc3. A lot of cleanup has
>    happened there lately.
>  - netif_stop_queue() is called earlier in _netdev_close().
>  - Minor cleanup: More helpful strings and return values.
>
> ---
>  .../networking/device_drivers/can/can327.rst  |  325 +++++
>  .../networking/device_drivers/can/index.rst   |    1 +
>  MAINTAINERS                                   |    7 +
>  drivers/net/can/Kconfig                       |   17 +
>  drivers/net/can/Makefile                      |    1 +
>  drivers/net/can/can327.c                      | 1158 +++++++++++++++++
>  include/uapi/linux/tty.h                      |    3 +-
>  7 files changed, 1511 insertions(+), 1 deletion(-)
>  create mode 100644 Documentation/networking/device_drivers/can/can327.rst
>  create mode 100644 drivers/net/can/can327.c
>
> diff --git a/Documentation/networking/device_drivers/can/can327.rst b/Documentation/networking/device_drivers/can/can327.rst
> new file mode 100644
> index 000000000000..48508963dd1b
> --- /dev/null
> +++ b/Documentation/networking/device_drivers/can/can327.rst
> @@ -0,0 +1,325 @@
> +.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause)
> +
> +can327: ELM327 driver for Linux SocketCAN
> +==========================================
> +
> +Authors
> +--------
> +
> +Max Staudt <max@enpas.org>
> +
> +
> +
> +Motivation
> +-----------
> +
> +This driver aims to lower the initial cost for hackers interested in
> +working with CAN buses.
> +
> +CAN adapters are expensive, few, and far between.
> +ELM327 interfaces are cheap and plentiful.
> +Let's use ELM327s as CAN adapters.
> +
> +
> +

Not sure if there is a convention of how many empty lines there should
be before a new section in a .rst file. However, three new lines seems
above average (had a quick peek, other .rst files usually use one or
two new lines).

> +Introduction
> +-------------
> +
> +This driver is an effort to turn abundant ELM327 based OBD interfaces
> +into full fledged (as far as possible) CAN interfaces.
> +
> +Since the ELM327 was never meant to be a stand alone CAN controller,
> +the driver has to switch between its modes as quickly as possible in
> +order to fake full-duplex operation.
> +
> +As such, can327 is a best effort driver. However, this is more than
> +enough to implement simple request-response protocols (such as OBD II),
> +and to monitor broadcast messages on a bus (such as in a vehicle).
> +
> +Most ELM327s come as nondescript serial devices, attached via USB or
> +Bluetooth. The driver cannot recognize them by itself, and as such it
> +is up to the user to attach it in form of a TTY line discipline
> +(similar to PPP, SLIP, slcan, ...).
> +
> +This driver is meant for ELM327 versions 1.4b and up, see below for
> +known limitations in older controllers and clones.
> +
> +
> +
> +Data sheet
> +-----------
> +
> +The official data sheets can be found at ELM electronics' home page:
> +
> +  https://www.elmelectronics.com/
> +
> +
> +
> +How to check the controller version
> +------------------------------------
> +
> +Use a terminal program to attach to the controller.
> +The default settings are 38400 baud/s, 8 data bits, no parity, 1 stopbit.
> +
> +After issuing the "``AT WS``" command, the controller will respond with
> +its version::
> +
> +    >AT WS
> +
> +
> +    ELM327 v1.4b
> +
> +    >
> +
> +Note that clones may claim to be any version they like.
> +It is not indicative of their actual feature set.
> +
> +
> +
> +How to attach the line discipline
> +----------------------------------
> +
> +Every ELM327 chip is factory programmed to operate at a serial setting
> +of 38400 baud/s, 8 data bits, no parity, 1 stopbit.

Information about baudrate parameters is given twice. You could
combine the two paragraphs not to repeat yourself.

> +The line discipline can be attached on a command prompt as follows::
> +
> +    sudo ldattach \
> +           --debug \
> +           --speed 38400 \
> +           --eightbits \
> +           --noparity \
> +           --onestopbit \
> +           --iflag -ICRNL,INLCR,-IXOFF \
> +           30 \
> +           /dev/ttyUSB0
> +
> +To change the ELM327's serial settings, please refer to its data
> +sheet. This needs to be done before attaching the line discipline.
> +
> +Once the ldisc is attached, the CAN interface starts out unconfigured.
> +Set the speed before starting it:

Shouldn't you use a double column :: here to indicate that the next
block is a piece of code?

> +    # The interface needs to be down to change parameters
> +    sudo ip link set can0 down
> +    sudo ip link set can0 type can bitrate 500000
> +    sudo ip link set can0 up
> +
> +500000 bit/s is a common rate for OBD-II diagnostics.
> +If you're connecting straight to a car's OBD port, this is the speed
> +that most cars (but not all!) expect.
> +
> +After this, you can set out as usual with candump, cansniffer, etc.
> +
> +
> +
> +Known limitations of the controller
> +------------------------------------
> +
> +- Clone devices ("v1.5" and others)
> +
> +  Sending RTR frames is not supported and will be dropped silently.
> +
> +  Receiving RTR with DLC 8 will appear to be a regular frame with
> +  the last received frame's DLC and payload.
> +
> +  "``AT CSM``" not supported, thus no ACK-ing frames while listening:
> +  "``AT MA``" will always be silent. However, immediately after
> +  sending a frame, the ELM327 will be in "receive reply" mode, in
> +  which it *does* ACK any received frames. Once the bus goes silent
> +  or an error occurs (such as BUFFER FULL), the ELM327 will end reply
> +  reception mode on its own and can327 will fall back to "``AT MA``"
> +  in order to keep monitoring the bus.

Maybe two sentences of what is an AT command would be helpful? It is
the Hayes command set, isn't it?
https://en.wikipedia.org/wiki/Hayes_command_set

Also, a quick memo of what the acronym CSM and MA mean would be great
(so that we do not need to have to constantly cross check the
datasheet).

> +
> +- All versions
> +
> +  No full duplex operation is supported. The driver will switch
> +  between input/output mode as quickly as possible.
> +
> +  The length of outgoing RTR frames cannot be set. In fact, some
> +  clones (tested with one identifying as "``v1.5``") are unable to
"> +  send RTR frames at all.
> +
> +  We don't have a way to get real-time notifications on CAN errors.
> +  While there is a command (``AT CS``) to retrieve some basic stats,
> +  we don't poll it as it would force us to interrupt reception mode.
> +
> +
> +- Versions prior to 1.4b
> +
> +  These versions do not send CAN ACKs when in monitoring mode (AT MA).
> +  However, they do send ACKs while waiting for a reply immediately
> +  after sending a frame. The driver maximizes this time to make the
> +  controller as useful as possible.
> +
> +  Starting with version 1.4b, the ELM327 supports the "``AT CSM``"
> +  command, and the "listen-only" CAN option will take effect.
> +
> +
> +- Versions prior to 1.4
> +
> +  These chips do not support the "``AT PB``" command, and thus cannot
> +  change bitrate or SFF/EFF mode on-the-fly. This will have to be
> +  programmed by the user before attaching the line discipline. See the
> +  data sheet for details.
> +
> +
> +- Versions prior to 1.3
> +
> +  These chips cannot be used at all with can327. They do not support
> +  the "``AT D1``" command, which is necessary to avoid parsing conflicts
> +  on incoming data, as well as distinction of RTR frame lengths.
> +
> +  Specifically, this allows for easy distinction of SFF and EFF
> +  frames, and to check whether frames are complete. While it is possible
> +  to deduce the type and length from the length of the line the ELM327
> +  sends us, this method fails when the ELM327's UART output buffer
> +  overruns. It may abort sending in the middle of the line, which will
> +  then be mistaken for something else.

Nitpick but I would prefer ascending order: 1.3 then 1.4, then 1.4b and so on.

> +
> +
> +Known limitations of the driver
> +--------------------------------
> +
> +- No 8/7 timing.
> +
> +  ELM327 can only set CAN bitrates that are of the form 500000/n, where
> +  n is an integer divisor.
> +  However there is an exception: With a separate flag, it may set the
> +  speed to be 8/7 of the speed indicated by the divisor.
> +  This mode is not currently implemented.
> +
> +- No evaluation of command responses.
> +
> +  The ELM327 will reply with OK when a command is understood, and with ?
> +  when it is not. The driver does not currently check this, and simply
> +  assumes that the chip understands every command.
> +  The driver is built such that functionality degrades gracefully
> +  nevertheless. See the section on known limitations of the controller.
> +
> +- No use of hardware CAN ID filtering
> +
> +  An ELM327's UART sending buffer will easily overflow on heavy CAN bus
> +  load, resulting in the "``BUFFER FULL``" message. Using the hardware
> +  filters available through "``AT CF xxx``" and "``AT CM xxx``" would be
> +  helpful here, however SocketCAN does not currently provide a facility
> +  to make use of such hardware features.
> +
> +
> +
> +Communication example
> +----------------------
> +
> +This is a short and incomplete introduction on how to talk to an ELM327.
> +
> +
> +The ELM327 has two modes:
> +
> +- Command mode
> +- Reception mode
> +
> +In command mode, it expects one command per line, terminated by CR.
> +By default, the prompt is a "``>``", after which a command can be
> +entered::
> +
> +    >ATE1
> +    OK
> +    >
> +
> +The init script in the driver switches off several configuration options
> +that are only meaningful in the original OBD scenario the chip is meant
> +for, and are actually a hindrance for can327.
> +
> +
> +When a command is not recognized, such as by an older version of the
> +ELM327, a question mark is printed as a response instead of OK::
> +
> +    >ATUNKNOWN
> +    ?
> +    >
> +
> +At present, can327 does not evaluate this response and silently assumes
> +that all commands are recognized. It is structured such that it will
> +degrade gracefully when a command is unknown. See the sections above on
> +known limitations for details.

This information is repeted twice whithin a dozen of lines. When I
read it, it was still fresh in my memory. Removing this paragraph
won't hurt, I think.

> +When a CAN frame is to be sent, the target address is configured, after
> +which the frame is sent as a command that consists of the data's hex
> +dump::
> +
> +    >ATSH123
> +    OK
> +    >DEADBEEF12345678
> +    OK
> +    >
> +
> +The above interaction sends the frame "``DE AD BE EF 12 34 56 78``" with
> +the 11 bit CAN ID ``0x123``.

Use the standard terminology:
11 bit CAN ID -> SFF (standard frame format)

> +For this to function, the controller must be configured for 11 bit CAN
> +ID sending mode (using "``AT PB``", see code or datasheet).
> +
> +
> +Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
> +configured on ``listen-only=off``), or when the reply timeout expires and
> +the driver sets the controller into monitoring mode (``ATMA``), the ELM327
> +will send one line for each received CAN frame, consisting of CAN ID,
> +DLC, and data::
> +
> +    123 8 DEADBEEF12345678
> +
> +For 29 bit CAN frames, the address format is slightly different, which

Use the standard terminology:
29 bit CAN frames -> EFF (extended frame format)

> +can327 uses to tell the two apart::
> +
> +    12 34 56 78 8 DEADBEEF12345678
> +
> +The ELM327 will receive both 11 and 29 bit frames - the current CAN
> +config (``ATPB``) does not matter.
> +
> +
> +If the ELM327's internal UART sending buffer runs full, it will abort
> +the monitoring mode, print "BUFFER FULL" and drop back into command
> +mode. Note that in this case, unlike with other error messages, the
> +error message may appear on the same line as the last (usually
> +incomplete) data frame::
> +
> +    12 34 56 78 8 DEADBEEF123 BUFFER FULL

Would be beneficial to put this section before the "known limitations"
one. It gives some nice context and help to better understand the
topics raised and reading this first really

> +
> +
> +Rationale behind the chosen configuration
> +------------------------------------------
> +
> +``AT E1``
> +  Echo on
> +
> +  We need this to be able to get a prompt reliably.
> +
> +``AT S1``
> +  Spaces on
> +
> +  We need this to distinguish 11/29 bit CAN addresses received.
> +
> +  Note:
> +  We can usually do this using the line length (odd/even),
> +  but this fails if the line is not transmitted fully to
> +  the host (BUFFER FULL).
> +
> +``AT D1``
> +  DLC on
> +
> +  We need this to tell the "length" of RTR frames.
> +
> +
> +
> +A note on CAN bus termination
> +------------------------------
> +
> +Your adapter may have resistors soldered in which are meant to terminate
> +the bus. This is correct when it is plugged into a OBD-II socket, but
> +not helpful when trying to tap into the middle of an existing CAN bus.
> +
> +If communications don't work with the adapter connected, check for the
> +termination resistors on its PCB and try removing them.
> diff --git a/Documentation/networking/device_drivers/can/index.rst b/Documentation/networking/device_drivers/can/index.rst
> index 58b6e0ad3030..c4f724db4908 100644
> --- a/Documentation/networking/device_drivers/can/index.rst
> +++ b/Documentation/networking/device_drivers/can/index.rst
> @@ -10,6 +10,7 @@ Contents:
>  .. toctree::
>     :maxdepth: 2
>
> +   can327
>     freescale/flexcan

This part of the patch does not apply. It conflicts with
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=5f02ecbe08d6a3434a14b92bee98adec76e8808e

Please rebase on the latest version on net-next.

>  .. only::  subproject and html
> diff --git a/MAINTAINERS b/MAINTAINERS
> index edc96cdb85e8..ab7f0906f07b 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -7218,6 +7218,13 @@ L:       netdev@vger.kernel.org
>  S:     Maintained
>  F:     drivers/net/ethernet/ibm/ehea/
>
> +ELM327 CAN NETWORK DRIVER
> +M:     Max Staudt <max@enpas.org>
> +L:     linux-can@vger.kernel.org
> +S:     Maintained
> +F:     Documentation/networking/device_drivers/can/can327.rst
> +F:     drivers/net/can/can327.c
> +
>  EM28XX VIDEO4LINUX DRIVER
>  M:     Mauro Carvalho Chehab <mchehab@kernel.org>
>  L:     linux-media@vger.kernel.org
> diff --git a/drivers/net/can/Kconfig b/drivers/net/can/Kconfig
> index fff259247d52..7900f9852e03 100644
> --- a/drivers/net/can/Kconfig
> +++ b/drivers/net/can/Kconfig
> @@ -180,6 +180,23 @@ source "drivers/net/can/softing/Kconfig"
>  source "drivers/net/can/spi/Kconfig"
>  source "drivers/net/can/usb/Kconfig"
>
> +config CAN_CAN327
> +       tristate "Serial / USB serial ELM327 based OBD-II Interfaces (can327)"
> +       depends on TTY
> +       help
> +         CAN driver for several 'low cost' OBD-II interfaces based on the
> +         ELM327 OBD-II interpreter chip.
> +
> +         This is a best effort driver - the ELM327 interface was never
> +         designed to be used as a standalone CAN interface. However, it can
> +         still be used for simple request-response protocols (such as OBD II),
> +         and to monitor broadcast messages on a bus (such as in a vehicle).
> +
> +         Please refer to the documentation for information on how to use it:
> +         Documentation/networking/device_drivers/can/can327.rst
> +
> +         If this driver is built as a module, it will be called can327.
> +
>  endif
>
>  config CAN_DEBUG_DEVICES
> diff --git a/drivers/net/can/Makefile b/drivers/net/can/Makefile
> index 1e660afcb61b..ff53bc274ae1 100644
> --- a/drivers/net/can/Makefile
> +++ b/drivers/net/can/Makefile
> @@ -6,6 +6,7 @@
>  obj-$(CONFIG_CAN_VCAN)         += vcan.o
>  obj-$(CONFIG_CAN_VXCAN)                += vxcan.o
>  obj-$(CONFIG_CAN_SLCAN)                += slcan.o
> +obj-$(CONFIG_CAN_CAN327)       += can327.o
>
>  obj-y                          += dev/
>  obj-y                          += rcar/
> diff --git a/drivers/net/can/can327.c b/drivers/net/can/can327.c
> new file mode 100644
> index 000000000000..e29031ad825d
> --- /dev/null
> +++ b/drivers/net/can/can327.c
> @@ -0,0 +1,1158 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* ELM327 based CAN interface driver (tty line discipline)
> + *
> + * This driver started as a derivative of linux/drivers/net/can/slcan.c
> + * and my thanks go to the original authors for their inspiration.
> + *
> + * can327.c Author : Max Staudt <max-linux@enpas.org>
> + * slcan.c Author  : Oliver Hartkopp <socketcan@hartkopp.net>
> + * slip.c Authors  : Laurence Culhane <loz@holmes.demon.co.uk>
> + *                   Fred N. van Kempen <waltje@uwalt.nl.mugnet.org>
> + */
> +
> +#define pr_fmt(fmt) "can327: " fmt
> +
> +#include <linux/init.h>
> +#include <linux/module.h>
> +#include <linux/moduleparam.h>

linux/moduleparam.h is not needed, right? Please make sure to clean up
unused headers.

> +
> +#include <linux/atomic.h>
> +#include <linux/bitops.h>
> +#include <linux/ctype.h>
> +#include <linux/delay.h>
> +#include <linux/errno.h>
> +#include <linux/if_ether.h>
> +#include <linux/kernel.h>
> +#include <linux/list.h>
> +#include <linux/lockdep.h>
> +#include <linux/netdevice.h>
> +#include <linux/skbuff.h>
> +#include <linux/spinlock.h>
> +#include <linux/string.h>
> +#include <linux/tty.h>
> +#include <linux/tty_ldisc.h>
> +#include <linux/workqueue.h>
> +
> +#include <uapi/linux/tty.h>
> +
> +#include <linux/can.h>
> +#include <linux/can/dev.h>
> +#include <linux/can/error.h>
> +#include <linux/can/led.h>

Rebase on net-next (or can-next) and remove CAN LED. c.f.:
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=6c1e423a3c84953edcf91ff03ab97829b287184a

> +#include <linux/can/rx-offload.h>
> +
> +#define ELM327_NAPI_WEIGHT 4
> +
> +#define ELM327_SIZE_RXBUF 224
> +#define ELM327_SIZE_TXBUF 32
> +
> +#define ELM327_CAN_CONFIG_SEND_SFF           0x8000
> +#define ELM327_CAN_CONFIG_VARIABLE_DLC       0x4000
> +#define ELM327_CAN_CONFIG_RECV_BOTH_SFF_EFF  0x2000
> +#define ELM327_CAN_CONFIG_BAUDRATE_MULT_8_7  0x1000
> +
> +#define ELM327_DUMMY_CHAR 'y'
> +#define ELM327_DUMMY_STRING "y"
> +#define ELM327_READY_CHAR '>'
> +
> +/* Bits in elm->cmds_todo */
> +enum elm327_tx_do {
> +       ELM327_TX_DO_CAN_DATA = 0,
> +       ELM327_TX_DO_CANID_11BIT,
> +       ELM327_TX_DO_CANID_29BIT_LOW,
> +       ELM327_TX_DO_CANID_29BIT_HIGH,
> +       ELM327_TX_DO_CAN_CONFIG_PART2,
> +       ELM327_TX_DO_CAN_CONFIG,
> +       ELM327_TX_DO_RESPONSES,
> +       ELM327_TX_DO_SILENT_MONITOR,
> +       ELM327_TX_DO_INIT
> +};
> +
> +struct can327 {
> +       /* This must be the first member when using alloc_candev() */
> +       struct can_priv can;
> +
> +       struct can_rx_offload offload;
> +
> +       /* TTY buffers */
> +       u8 rxbuf[ELM327_SIZE_RXBUF];
> +       u8 txbuf[ELM327_SIZE_TXBUF];
> +
> +       /* Per-channel lock */
> +       spinlock_t lock;
> +
> +       /* TTY and netdev devices that we're bridging */
> +       struct tty_struct *tty;
> +       struct net_device *dev;
> +
> +       /* TTY buffer accounting */
> +       struct work_struct tx_work;     /* Flushes TTY TX buffer */
> +       u8 *txhead;                     /* Next TX byte */
> +       size_t txleft;                  /* Bytes left to TX */
> +       int rxfill;                     /* Bytes already RX'd in buffer */
> +
> +       /* State machine */
> +       enum {
> +               ELM327_STATE_NOTINIT = 0,
> +               ELM327_STATE_GETDUMMYCHAR,
> +               ELM327_STATE_GETPROMPT,
> +               ELM327_STATE_RECEIVING,
> +       } state;
> +
> +       /* Things we have yet to send */
> +       char **next_init_cmd;
> +       unsigned long cmds_todo;
> +
> +       /* The CAN frame and config the ELM327 is sending/using,
> +        * or will send/use after finishing all cmds_todo
> +        */
> +       struct can_frame can_frame_to_send;
> +       u16 can_config;
> +       u8 can_bitrate_divisor;
> +
> +       /* Parser state */
> +       bool drop_next_line;
> +
> +       /* Stop the channel on UART side hardware failure, e.g. stray
> +        * characters or neverending lines. This may be caused by bad
> +        * UART wiring, a bad ELM327, a bad UART bridge...
> +        * Once this is true, nothing will be sent to the TTY.
> +        */
> +       bool uart_side_failure;
> +};
> +
> +static inline void elm327_uart_side_failure(struct can327 *elm);
> +
> +static void elm327_send(struct can327 *elm, const void *buf, size_t len)
> +{
> +       int written;
> +
> +       lockdep_assert_held(&elm->lock);
> +
> +       if (elm->uart_side_failure)
> +               return;
> +
> +       memcpy(elm->txbuf, buf, len);
> +
> +       /* Order of next two lines is *very* important.
> +        * When we are sending a little amount of data,
> +        * the transfer may be completed inside the ops->write()
> +        * routine, because it's running with interrupts enabled.
> +        * In this case we *never* got WRITE_WAKEUP event,
> +        * if we did not request it before write operation.
> +        *       14 Oct 1994  Dmitry Gorodchanin.
> +        */
> +       set_bit(TTY_DO_WRITE_WAKEUP, &elm->tty->flags);
> +       written = elm->tty->ops->write(elm->tty, elm->txbuf, len);
> +       if (written < 0) {
> +               netdev_err(elm->dev,
> +                          "Failed to write to tty %s.\n",
> +                          elm->tty->name);
> +               elm327_uart_side_failure(elm);
> +               return;
> +       }
> +
> +       elm->txleft = len - written;
> +       elm->txhead = elm->txbuf + written;
> +}
> +
> +/* Take the ELM327 out of almost any state and back into command mode.
> + * We send ELM327_DUMMY_CHAR which will either abort any running
> + * operation, or be echoed back to us in case we're already in command
> + * mode.
> + */
> +static void elm327_kick_into_cmd_mode(struct can327 *elm)
> +{
> +       lockdep_assert_held(&elm->lock);
> +
> +       if (elm->state != ELM327_STATE_GETDUMMYCHAR &&
> +           elm->state != ELM327_STATE_GETPROMPT) {
> +               elm327_send(elm, ELM327_DUMMY_STRING, 1);
> +
> +               elm->state = ELM327_STATE_GETDUMMYCHAR;
> +       }
> +}
> +
> +/* Schedule a CAN frame and necessary config changes to be sent to the TTY. */
> +static void elm327_send_frame(struct can327 *elm, struct can_frame *frame)
> +{
> +       lockdep_assert_held(&elm->lock);
> +
> +       /* Schedule any necessary changes in ELM327's CAN configuration */
> +       if (elm->can_frame_to_send.can_id != frame->can_id) {
> +               /* Set the new CAN ID for transmission. */
> +               if ((frame->can_id ^ elm->can_frame_to_send.can_id)
> +                   & CAN_EFF_FLAG) {
> +                       elm->can_config = (frame->can_id & CAN_EFF_FLAG
> +                                               ? 0
> +                                               : ELM327_CAN_CONFIG_SEND_SFF)
> +                                       | ELM327_CAN_CONFIG_VARIABLE_DLC
> +                                       | ELM327_CAN_CONFIG_RECV_BOTH_SFF_EFF
> +                                       | elm->can_bitrate_divisor;
> +
> +                       set_bit(ELM327_TX_DO_CAN_CONFIG, &elm->cmds_todo);
> +               }
> +
> +               if (frame->can_id & CAN_EFF_FLAG) {
> +                       clear_bit(ELM327_TX_DO_CANID_11BIT, &elm->cmds_todo);
> +                       set_bit(ELM327_TX_DO_CANID_29BIT_LOW, &elm->cmds_todo);
> +                       set_bit(ELM327_TX_DO_CANID_29BIT_HIGH, &elm->cmds_todo);
> +               } else {
> +                       set_bit(ELM327_TX_DO_CANID_11BIT, &elm->cmds_todo);
> +                       clear_bit(ELM327_TX_DO_CANID_29BIT_LOW, &elm->cmds_todo);
> +                       clear_bit(ELM327_TX_DO_CANID_29BIT_HIGH, &elm->cmds_todo);
> +               }
> +       }
> +
> +       /* Schedule the CAN frame itself. */
> +       elm->can_frame_to_send = *frame;
> +       set_bit(ELM327_TX_DO_CAN_DATA, &elm->cmds_todo);
> +
> +       elm327_kick_into_cmd_mode(elm);
> +}
> +
> +/* ELM327 initialisation sequence.
> + * The line length is limited by the buffer in elm327_handle_prompt().
> + */
> +static char *elm327_init_script[] = {
> +       "AT WS\r",        /* v1.0: Warm Start */
> +       "AT PP FF OFF\r", /* v1.0: All Programmable Parameters Off */
> +       "AT M0\r",        /* v1.0: Memory Off */
> +       "AT AL\r",        /* v1.0: Allow Long messages */
> +       "AT BI\r",        /* v1.0: Bypass Initialisation */
> +       "AT CAF0\r",      /* v1.0: CAN Auto Formatting Off */
> +       "AT CFC0\r",      /* v1.0: CAN Flow Control Off */
> +       "AT CF 000\r",    /* v1.0: Reset CAN ID Filter */
> +       "AT CM 000\r",    /* v1.0: Reset CAN ID Mask */
> +       "AT E1\r",        /* v1.0: Echo On */
> +       "AT H1\r",        /* v1.0: Headers On */
> +       "AT L0\r",        /* v1.0: Linefeeds Off */
> +       "AT SH 7DF\r",    /* v1.0: Set CAN sending ID to 0x7df */
> +       "AT ST FF\r",     /* v1.0: Set maximum Timeout for response after TX */
> +       "AT AT0\r",       /* v1.2: Adaptive Timing Off */
> +       "AT D1\r",        /* v1.3: Print DLC On */
> +       "AT S1\r",        /* v1.3: Spaces On */
> +       "AT TP B\r",      /* v1.0: Try Protocol B */
> +       NULL
> +};
> +
> +static void elm327_init(struct can327 *elm)
> +{
> +       lockdep_assert_held(&elm->lock);
> +
> +       elm->state = ELM327_STATE_NOTINIT;
> +       elm->can_frame_to_send.can_id = 0x7df; /* ELM327 HW default */
> +       elm->rxfill = 0;
> +       elm->drop_next_line = 0;
> +
> +       /* We can only set the bitrate as a fraction of 500000.
> +        * The bit timing constants in can327_bittiming_const will
> +        * limit the user to the right values.
> +        */
> +       elm->can_bitrate_divisor = 500000 / elm->can.bittiming.bitrate;
> +       elm->can_config = ELM327_CAN_CONFIG_SEND_SFF
> +                       | ELM327_CAN_CONFIG_VARIABLE_DLC
> +                       | ELM327_CAN_CONFIG_RECV_BOTH_SFF_EFF
> +                       | elm->can_bitrate_divisor;
> +
> +       /* Configure ELM327 and then start monitoring */
> +       elm->next_init_cmd = &elm327_init_script[0];
> +       set_bit(ELM327_TX_DO_INIT, &elm->cmds_todo);
> +       set_bit(ELM327_TX_DO_SILENT_MONITOR, &elm->cmds_todo);
> +       set_bit(ELM327_TX_DO_RESPONSES, &elm->cmds_todo);
> +       set_bit(ELM327_TX_DO_CAN_CONFIG, &elm->cmds_todo);
> +
> +       elm327_kick_into_cmd_mode(elm);
> +}
> +
> +static void elm327_feed_frame_to_netdev(struct can327 *elm,
> +                                       struct sk_buff *skb)
> +{
> +       lockdep_assert_held(&elm->lock);
> +
> +       if (!netif_running(elm->dev))
> +               return;
> +
> +       /* Queue for NAPI pickup.
> +        * rx-offload will update stats and LEDs for us.
> +        */
> +       if (can_rx_offload_queue_tail(&elm->offload, skb))
> +               elm->dev->stats.rx_fifo_errors++;
> +
> +       /* Wake NAPI */
> +       can_rx_offload_irq_finish(&elm->offload);
> +}
> +
> +/* Called when we're out of ideas and just want it all to end. */
> +static inline void elm327_uart_side_failure(struct can327 *elm)
> +{
> +       struct can_frame *frame;
> +       struct sk_buff *skb;
> +
> +       lockdep_assert_held(&elm->lock);
> +
> +       elm->uart_side_failure = true;
> +
> +       clear_bit(TTY_DO_WRITE_WAKEUP, &elm->tty->flags);
> +
> +       elm->can.can_stats.bus_off++;
> +       netif_stop_queue(elm->dev);
> +       elm->can.state = CAN_STATE_BUS_OFF;
> +       can_bus_off(elm->dev);
> +
> +       netdev_err(elm->dev, "ELM327 misbehaved. Blocking further communication.\n");
> +
> +       skb = alloc_can_err_skb(elm->dev, &frame);
> +       if (!skb)
> +               return;
> +
> +       frame->can_id |= CAN_ERR_BUSOFF;
> +       elm327_feed_frame_to_netdev(elm, skb);
> +}
> +
> +/* Compares a byte buffer (non-NUL terminated) to the payload part of a string,
> + * and returns true iff the buffer (content *and* length) is exactly that
> + * string, without the terminating NUL byte.
> + *
> + * Example: If reference is "BUS ERROR", then this returns true iff nbytes == 9
> + *          and !memcmp(buf, "BUS ERROR", 9).
> + *
> + * The reason to use strings is so we can easily include them in the C code,
> + * and to avoid hardcoding lengths.
> + */
> +static inline bool elm327_rxbuf_cmp(const u8 *buf, size_t nbytes, const char *reference)
> +{
> +       size_t ref_len = strlen(reference);
> +
> +       return (nbytes == ref_len) && !memcmp(buf, reference, ref_len);
> +}
> +
> +static void elm327_parse_error(struct can327 *elm, size_t len)
> +{
> +       struct can_frame *frame;
> +       struct sk_buff *skb;
> +
> +       lockdep_assert_held(&elm->lock);
> +
> +       skb = alloc_can_err_skb(elm->dev, &frame);
> +       if (!skb)
> +               /* It's okay to return here:
> +                * The outer parsing loop will drop this UART buffer.
> +                */
> +               return;
> +
> +       /* Filter possible error messages based on length of RX'd line */
> +       if (elm327_rxbuf_cmp(elm->rxbuf, len, "UNABLE TO CONNECT")) {
> +               netdev_err(elm->dev,
> +                          "ELM327 reported UNABLE TO CONNECT. Please check your setup.\n");
> +       } else if (elm327_rxbuf_cmp(elm->rxbuf, len, "BUFFER FULL")) {
> +               /* This will only happen if the last data line was complete.
> +                * Otherwise, elm327_parse_frame() will heuristically
> +                * emit this kind of error frame instead.
> +                */
> +               frame->can_id |= CAN_ERR_CRTL;
> +               frame->data[1] = CAN_ERR_CRTL_RX_OVERFLOW;
> +       } else if (elm327_rxbuf_cmp(elm->rxbuf, len, "BUS ERROR")) {
> +               frame->can_id |= CAN_ERR_BUSERROR;
> +       } else if (elm327_rxbuf_cmp(elm->rxbuf, len, "CAN ERROR")) {
> +               frame->can_id |= CAN_ERR_PROT;
> +       } else if (elm327_rxbuf_cmp(elm->rxbuf, len, "<RX ERROR")) {
> +               frame->can_id |= CAN_ERR_PROT;
> +       } else if (elm327_rxbuf_cmp(elm->rxbuf, len, "BUS BUSY")) {
> +               frame->can_id |= CAN_ERR_PROT;
> +               frame->data[2] = CAN_ERR_PROT_OVERLOAD;
> +       } else if (elm327_rxbuf_cmp(elm->rxbuf, len, "FB ERROR")) {
> +               frame->can_id |= CAN_ERR_PROT;
> +               frame->data[2] = CAN_ERR_PROT_TX;
> +       } else if (len == 5 && !memcmp(elm->rxbuf, "ERR", 3)) {
> +               /* ERR is followed by two digits, hence line length 5 */
> +               netdev_err(elm->dev, "ELM327 reported an ERR%c%c. Please power it off and on again.\n",
> +                          elm->rxbuf[3], elm->rxbuf[4]);
> +               frame->can_id |= CAN_ERR_CRTL;
> +       } else {
> +               /* Something else has happened.
> +                * Maybe garbage on the UART line.
> +                * Emit a generic error frame.
> +                */
> +       }
> +
> +       elm327_feed_frame_to_netdev(elm, skb);
> +}
> +
> +/* Parse CAN frames coming as ASCII from ELM327.
> + * They can be of various formats:
> + *
> + * 29-bit ID (EFF):  12 34 56 78 D PL PL PL PL PL PL PL PL
> + * 11-bit ID (!EFF): 123 D PL PL PL PL PL PL PL PL
> + *
> + * where D = DLC, PL = payload byte
> + *
> + * Instead of a payload, RTR indicates a remote request.
> + *
> + * We will use the spaces and line length to guess the format.
> + */
> +static int elm327_parse_frame(struct can327 *elm, size_t len)
> +{
> +       struct can_frame *frame;
> +       struct sk_buff *skb;
> +       int hexlen;
> +       int datastart;
> +       int i;
> +
> +       lockdep_assert_held(&elm->lock);
> +
> +       skb = alloc_can_skb(elm->dev, &frame);
> +       if (!skb)
> +               return -ENOMEM;
> +
> +       /* Find first non-hex and non-space character:
> +        *  - In the simplest case, there is none.
> +        *  - For RTR frames, 'R' is the first non-hex character.
> +        *  - An error message may replace the end of the data line.
> +        */
> +       for (hexlen = 0; hexlen <= len; hexlen++) {
> +               if (hex_to_bin(elm->rxbuf[hexlen]) < 0 &&
> +                   elm->rxbuf[hexlen] != ' ') {
> +                       break;
> +               }
> +       }
> +
> +       /* Sanity check whether the line is really a clean hexdump,
> +        * or terminated by an error message, or contains garbage.
> +        */
> +       if (hexlen < len &&
> +           !isdigit(elm->rxbuf[hexlen]) &&
> +           !isupper(elm->rxbuf[hexlen]) &&
> +           '<' != elm->rxbuf[hexlen] &&
> +           ' ' != elm->rxbuf[hexlen]) {
> +               /* The line is likely garbled anyway, so bail.
> +                * The main code will restart listening.
> +                */
> +               return -ENODATA;

Here (and on other return statement of this function), I think you
have a memory leak on skb. You probably need a goto label to free the
skb when error occurs.

> +       }
> +
> +       /* Use spaces in CAN ID to distinguish 29 or 11 bit address length.
> +        * No out-of-bounds access:
> +        * We use the fact that we can always read from elm->rxbuf.
> +        */
> +       if (elm->rxbuf[2] == ' ' && elm->rxbuf[5] == ' ' &&
> +           elm->rxbuf[8] == ' ' && elm->rxbuf[11] == ' ' &&
> +           elm->rxbuf[13] == ' ') {
> +               frame->can_id = CAN_EFF_FLAG;
> +               datastart = 14;
> +       } else if (elm->rxbuf[3] == ' ' && elm->rxbuf[5] == ' ') {
> +               datastart = 6;
> +       } else {
> +               /* This is not a well-formatted data line.
> +                * Assume it's an error message.
> +                */
> +               return -ENODATA;
> +       }
> +
> +       if (hexlen < datastart) {
> +               /* The line is too short to be a valid frame hex dump.
> +                * Something interrupted the hex dump or it is invalid.
> +                */
> +               return -ENODATA;
> +       }
> +
> +       /* From here on all chars up to buf[hexlen] are hex or spaces,
> +        * at well-defined offsets.
> +        */
> +
> +       /* Read CAN data length */
> +       frame->len = (hex_to_bin(elm->rxbuf[datastart - 2]) << 0);
> +
> +       /* Read CAN ID */
> +       if (frame->can_id & CAN_EFF_FLAG) {
> +               frame->can_id |= (hex_to_bin(elm->rxbuf[0]) << 28)
> +                              | (hex_to_bin(elm->rxbuf[1]) << 24)
> +                              | (hex_to_bin(elm->rxbuf[3]) << 20)
> +                              | (hex_to_bin(elm->rxbuf[4]) << 16)
> +                              | (hex_to_bin(elm->rxbuf[6]) << 12)
> +                              | (hex_to_bin(elm->rxbuf[7]) << 8)
> +                              | (hex_to_bin(elm->rxbuf[9]) << 4)
> +                              | (hex_to_bin(elm->rxbuf[10]) << 0);
> +       } else {
> +               frame->can_id |= (hex_to_bin(elm->rxbuf[0]) << 8)
> +                              | (hex_to_bin(elm->rxbuf[1]) << 4)
> +                              | (hex_to_bin(elm->rxbuf[2]) << 0);
> +       }
> +
> +       /* Check for RTR frame */
> +       if (elm->rxfill >= hexlen + 3 &&
> +           !memcmp(&elm->rxbuf[hexlen], "RTR", 3)) {
> +               frame->can_id |= CAN_RTR_FLAG;
> +       }
> +
> +       /* Is the line long enough to hold the advertised payload?
> +        * Note: RTR frames have a DLC, but no actual payload.
> +        */
> +       if (!(frame->can_id & CAN_RTR_FLAG) &&
> +           (hexlen < frame->len * 3 + datastart)) {
> +               /* Incomplete frame.
> +                * Probably the ELM327's RS232 TX buffer was full.
> +                * Emit an error frame and exit.
> +                */
> +               frame->can_id = CAN_ERR_FLAG | CAN_ERR_CRTL;
> +               frame->len = CAN_ERR_DLC;
> +               frame->data[1] = CAN_ERR_CRTL_RX_OVERFLOW;
> +               elm327_feed_frame_to_netdev(elm, skb);
> +
> +               /* Signal failure to parse.
> +                * The line will be re-parsed as an error line, which will fail.
> +                * However, this will correctly drop the state machine back into
> +                * command mode.
> +                */
> +               return -ENODATA;
> +       }
> +
> +       /* Parse the data nibbles. */
> +       for (i = 0; i < frame->len; i++) {
> +               frame->data[i] = (hex_to_bin(elm->rxbuf[datastart + 3*i]) << 4)
> +                              | (hex_to_bin(elm->rxbuf[datastart + 3*i + 1]));
> +       }
> +
> +       /* Feed the frame to the network layer. */
> +       elm327_feed_frame_to_netdev(elm, skb);
> +
> +       return 0;
> +}
> +
> +static void elm327_parse_line(struct can327 *elm, size_t len)
> +{
> +       lockdep_assert_held(&elm->lock);
> +
> +       /* Skip empty lines */
> +       if (!len)
> +               return;
> +
> +       /* Skip echo lines */
> +       if (elm->drop_next_line) {
> +               elm->drop_next_line = 0;
> +               return;
> +       } else if (!memcmp(elm->rxbuf, "AT", 2)) {
> +               return;
> +       }
> +
> +       /* Regular parsing */
> +       if (elm->state == ELM327_STATE_RECEIVING &&
> +           elm327_parse_frame(elm, len)) {
> +               /* Parse an error line. */
> +               elm327_parse_error(elm, len);
> +
> +               /* Start afresh. */
> +               elm327_kick_into_cmd_mode(elm);
> +       }
> +}
> +
> +static void elm327_handle_prompt(struct can327 *elm)
> +{
> +       struct can_frame *frame = &elm->can_frame_to_send;
> +       /* Size this buffer for the largest ELM327 line we may generate,
> +        * which is currently an 8 byte CAN frame's payload hexdump.
> +        * Items in elm327_init_script must fit here, too!
> +        */
> +       char local_txbuf[sizeof("0102030405060708\r")];
> +
> +       lockdep_assert_held(&elm->lock);
> +
> +       if (!elm->cmds_todo) {
> +               /* Enter CAN monitor mode */
> +               elm327_send(elm, "ATMA\r", 5);
> +               elm->state = ELM327_STATE_RECEIVING;
> +
> +               /* We will be in the default state once this command is
> +                * sent, so enable the TX packet queue.
> +                */
> +               netif_wake_queue(elm->dev);
> +
> +               return;
> +       }
> +
> +       /* Reconfigure ELM327 step by step as indicated by elm->cmds_todo */
> +       if (test_bit(ELM327_TX_DO_INIT, &elm->cmds_todo)) {
> +               snprintf(local_txbuf, sizeof(local_txbuf),
> +                        "%s",
> +                        *elm->next_init_cmd);
> +
> +               elm->next_init_cmd++;
> +               if (!(*elm->next_init_cmd)) {
> +                       clear_bit(ELM327_TX_DO_INIT, &elm->cmds_todo);
> +                       /* Init finished. */
> +               }
> +
> +       } else if (test_and_clear_bit(ELM327_TX_DO_SILENT_MONITOR, &elm->cmds_todo)) {
> +               snprintf(local_txbuf, sizeof(local_txbuf),
> +                        "ATCSM%i\r",
> +                        !!(elm->can.ctrlmode & CAN_CTRLMODE_LISTENONLY));
> +
> +       } else if (test_and_clear_bit(ELM327_TX_DO_RESPONSES, &elm->cmds_todo)) {
> +               snprintf(local_txbuf, sizeof(local_txbuf),
> +                        "ATR%i\r",
> +                        !(elm->can.ctrlmode & CAN_CTRLMODE_LISTENONLY));
> +
> +       } else if (test_and_clear_bit(ELM327_TX_DO_CAN_CONFIG, &elm->cmds_todo)) {
> +               snprintf(local_txbuf, sizeof(local_txbuf),
> +                        "ATPC\r");
> +               set_bit(ELM327_TX_DO_CAN_CONFIG_PART2, &elm->cmds_todo);
> +
> +       } else if (test_and_clear_bit(ELM327_TX_DO_CAN_CONFIG_PART2, &elm->cmds_todo)) {
> +               snprintf(local_txbuf, sizeof(local_txbuf),
> +                        "ATPB%04X\r",
> +                        elm->can_config);
> +
> +       } else if (test_and_clear_bit(ELM327_TX_DO_CANID_29BIT_HIGH, &elm->cmds_todo)) {
> +               snprintf(local_txbuf, sizeof(local_txbuf),
> +                        "ATCP%02X\r",
> +                        (frame->can_id & CAN_EFF_MASK) >> 24);
> +
> +       } else if (test_and_clear_bit(ELM327_TX_DO_CANID_29BIT_LOW, &elm->cmds_todo)) {
> +               snprintf(local_txbuf, sizeof(local_txbuf),
> +                        "ATSH%06X\r",
> +                        frame->can_id & CAN_EFF_MASK & ((1 << 24) - 1));
> +
> +       } else if (test_and_clear_bit(ELM327_TX_DO_CANID_11BIT, &elm->cmds_todo)) {
> +               snprintf(local_txbuf, sizeof(local_txbuf),
> +                        "ATSH%03X\r",
> +                        frame->can_id & CAN_SFF_MASK);
> +
> +       } else if (test_and_clear_bit(ELM327_TX_DO_CAN_DATA, &elm->cmds_todo)) {
> +               if (frame->can_id & CAN_RTR_FLAG) {
> +                       /* Send an RTR frame. Their DLC is fixed.
> +                        * Some chips don't send them at all.
> +                        */
> +                       snprintf(local_txbuf, sizeof(local_txbuf),
> +                                "ATRTR\r");
> +               } else {
> +                       /* Send a regular CAN data frame */
> +                       int i;
> +
> +                       for (i = 0; i < frame->len; i++) {
> +                               snprintf(&local_txbuf[2 * i], sizeof(local_txbuf),
> +                                        "%02X",
> +                                        frame->data[i]);
> +                       }
> +
> +                       snprintf(&local_txbuf[2 * i], sizeof(local_txbuf),
> +                                "\r");
> +               }
> +
> +               elm->drop_next_line = 1;
> +               elm->state = ELM327_STATE_RECEIVING;
> +
> +               /* We will be in the default state once this command is
> +                * sent, so enable the TX packet queue.
> +                */
> +               netif_wake_queue(elm->dev);
> +       }
> +
> +       elm327_send(elm, local_txbuf, strlen(local_txbuf));
> +}
> +
> +static bool elm327_is_ready_char(char c)
> +{
> +       /* Bits 0xc0 are sometimes set (randomly), hence the mask.
> +        * Probably bad hardware.
> +        */
> +       return (c & 0x3f) == ELM327_READY_CHAR;
> +}
> +
> +static void elm327_drop_bytes(struct can327 *elm, size_t i)
> +{
> +       lockdep_assert_held(&elm->lock);
> +
> +       memmove(&elm->rxbuf[0], &elm->rxbuf[i], ELM327_SIZE_RXBUF - i);
> +       elm->rxfill -= i;
> +}
> +
> +static void elm327_parse_rxbuf(struct can327 *elm, size_t first_new_char_idx)
> +{
> +       size_t len, pos;
> +
> +       lockdep_assert_held(&elm->lock);
> +
> +       switch (elm->state) {
> +       case ELM327_STATE_NOTINIT:
> +               elm->rxfill = 0;
> +               break;
> +
> +       case ELM327_STATE_GETDUMMYCHAR:
> +               /* Wait for 'y' or '>' */
> +               for (pos = 0; pos < elm->rxfill; pos++) {
> +                       if (elm->rxbuf[pos] == ELM327_DUMMY_CHAR) {
> +                               elm327_send(elm, "\r", 1);
> +                               elm->state = ELM327_STATE_GETPROMPT;
> +                               pos++;
> +                               break;
> +                       } else if (elm327_is_ready_char(elm->rxbuf[pos])) {
> +                               elm327_send(elm, ELM327_DUMMY_STRING, 1);
> +                               pos++;
> +                               break;
> +                       }
> +               }
> +
> +               elm327_drop_bytes(elm, pos);
> +               break;
> +
> +       case ELM327_STATE_GETPROMPT:
> +               /* Wait for '>' */
> +               if (elm327_is_ready_char(elm->rxbuf[elm->rxfill - 1]))
> +                       elm327_handle_prompt(elm);
> +
> +               elm->rxfill = 0;
> +               break;
> +
> +       case ELM327_STATE_RECEIVING:
> +               /* Find <CR> delimiting feedback lines. */
> +               len = first_new_char_idx;
> +               while (len < elm->rxfill && elm->rxbuf[len] != '\r')
> +                       len++;
> +
> +               if (len == ELM327_SIZE_RXBUF) {
> +                       /* Assume the buffer ran full with garbage.
> +                        * Did we even connect at the right baud rate?
> +                        */
> +                       netdev_err(elm->dev,
> +                                  "RX buffer overflow. Faulty ELM327 or UART?\n");
> +                       elm327_uart_side_failure(elm);
> +               } else if (len == elm->rxfill) {
> +                       if (elm327_is_ready_char(elm->rxbuf[elm->rxfill - 1])) {
> +                               /* The ELM327's AT ST response timeout ran out,
> +                                * so we got a prompt.
> +                                * Clear RX buffer and restart listening.
> +                                */
> +                               elm->rxfill = 0;
> +
> +                               elm327_handle_prompt(elm);
> +                       }
> +
> +                       /* No <CR> found - we haven't received a full line yet.
> +                        * Wait for more data.
> +                        */
> +               } else {
> +                       /* We have a full line to parse. */
> +                       elm327_parse_line(elm, len);
> +
> +                       /* Remove parsed data from RX buffer. */
> +                       elm327_drop_bytes(elm, len + 1);
> +
> +                       /* More data to parse? */
> +                       if (elm->rxfill)
> +                               elm327_parse_rxbuf(elm, 0);
> +               }
> +       }
> +}
> +
> +static int can327_netdev_open(struct net_device *dev)
> +{
> +       struct can327 *elm = netdev_priv(dev);
> +       int err;
> +
> +       spin_lock_bh(&elm->lock);
> +
> +       if (!elm->tty) {
> +               spin_unlock_bh(&elm->lock);
> +               return -ENODEV;
> +       }
> +
> +       if (elm->uart_side_failure)
> +               netdev_warn(elm->dev, "Reopening netdev after a UART side fault has been detected.\n");
> +
> +       /* Clear TTY buffers */
> +       elm->rxfill = 0;
> +       elm->txleft = 0;
> +
> +       /* open_candev() checks for elm->can.bittiming.bitrate != 0 */
> +       err = open_candev(dev);
> +       if (err) {
> +               spin_unlock_bh(&elm->lock);
> +               return err;
> +       }
> +
> +       elm327_init(elm);
> +       spin_unlock_bh(&elm->lock);
> +
> +       err = can_rx_offload_add_manual(dev, &elm->offload, ELM327_NAPI_WEIGHT);
> +       if (err) {
> +               close_candev(dev);
> +               return err;
> +       }
> +
> +       can_rx_offload_enable(&elm->offload);
> +
> +       can_led_event(dev, CAN_LED_EVENT_OPEN);
> +       elm->can.state = CAN_STATE_ERROR_ACTIVE;
> +       netif_start_queue(dev);
> +
> +       return 0;
> +}
> +
> +static int can327_netdev_close(struct net_device *dev)
> +{
> +       struct can327 *elm = netdev_priv(dev);
> +
> +       /* Interrupt whatever the ELM327 is doing right now */
> +       spin_lock_bh(&elm->lock);
> +       elm327_send(elm, ELM327_DUMMY_STRING, 1);
> +       spin_unlock_bh(&elm->lock);
> +
> +       netif_stop_queue(dev);
> +
> +       /* Give UART one final chance to flush. */
> +       clear_bit(TTY_DO_WRITE_WAKEUP, &elm->tty->flags);
> +       flush_work(&elm->tx_work);
> +
> +       can_rx_offload_disable(&elm->offload);
> +       elm->can.state = CAN_STATE_STOPPED;
> +       can_rx_offload_del(&elm->offload);
> +       close_candev(dev);
> +       can_led_event(dev, CAN_LED_EVENT_STOP);
> +
> +       return 0;
> +}
> +
> +/* Send a can_frame to a TTY. */
> +static netdev_tx_t can327_netdev_start_xmit(struct sk_buff *skb,
> +                                           struct net_device *dev)
> +{
> +       struct can327 *elm = netdev_priv(dev);
> +       struct can_frame *frame = (struct can_frame *)skb->data;
> +
> +       if (can_dropped_invalid_skb(dev, skb))
> +               return NETDEV_TX_OK;
> +
> +       /* This check will be part of can_dropped_invalid_skb()
> +        * in future kernels.
> +        */
> +       if (elm->can.ctrlmode & CAN_CTRLMODE_LISTENONLY)
> +               goto out;
> +
> +       /* We shouldn't get here after a hardware fault:
> +        * can_bus_off() calls netif_carrier_off()
> +        */
> +       if (elm->uart_side_failure) {
> +               WARN_ON_ONCE(elm->uart_side_failure);
> +               goto out;
> +       }
> +
> +       netif_stop_queue(dev);
> +
> +       /* BHs are already disabled, so no spin_lock_bh().
> +        * See Documentation/networking/netdevices.txt
> +        */
> +       spin_lock(&elm->lock);
> +       elm327_send_frame(elm, frame);
> +       spin_unlock(&elm->lock);
> +
> +       dev->stats.tx_packets++;
> +       dev->stats.tx_bytes += frame->len;
> +
> +       can_led_event(dev, CAN_LED_EVENT_TX);
> +
> +out:
> +       kfree_skb(skb);
> +       return NETDEV_TX_OK;

So here, you never loopback the skb. For what I understand, the
controller does not provide such loopback features.

However, local loopback is a core part of SocketCAN Concept, c.f.:
https://docs.kernel.org/networking/can.html#local-loopback-of-sent-frames

It is still better to loopback even if you can not know for sure if
the transmission is successfull.
The easiest way to loopback your skb by using can_put_echo_skb() and
then can_get_echo_skb() right away, similar to what cc770 does:
https://elixir.bootlin.com/linux/latest/source/drivers/net/can/cc770/cc770.c#L698

If you do so, do not forget to adjust your call to alloc_candev() to
add room for one echo skb.

> +}
> +
> +static const struct net_device_ops can327_netdev_ops = {
> +       .ndo_open       = can327_netdev_open,
> +       .ndo_stop       = can327_netdev_close,
> +       .ndo_start_xmit = can327_netdev_start_xmit,
> +       .ndo_change_mtu = can_change_mtu,
> +};
> +
> +static bool can327_is_valid_rx_char(u8 c)
> +{
> +       static const bool lut_char_is_valid['z'] = {
> +               ['\r'] = true,
> +               [' '] = true,
> +               ['.'] = true,
> +               ['0'] = true, true, true, true, true,
> +               ['5'] = true, true, true, true, true,
> +               ['<'] = true,
> +               [ELM327_READY_CHAR] = true,
> +               ['?'] = true,
> +               ['A'] = true, true, true, true, true, true, true,
> +               ['H'] = true, true, true, true, true, true, true,
> +               ['O'] = true, true, true, true, true, true, true,
> +               ['V'] = true, true, true, true, true,
> +               ['a'] = true,
> +               ['b'] = true,
> +               ['v'] = true,
> +               [ELM327_DUMMY_CHAR] = true,
> +       };
> +       BUILD_BUG_ON(ELM327_DUMMY_CHAR >= 'z');
> +
> +       return (c < ARRAY_SIZE(lut_char_is_valid) &&
> +               lut_char_is_valid[c]);
> +}
> +
> +/* Handle incoming ELM327 ASCII data.
> + * This will not be re-entered while running, but other ldisc
> + * functions may be called in parallel.
> + */
> +static void can327_ldisc_rx(struct tty_struct *tty,
> +                           const unsigned char *cp, const char *fp, int count)
> +{
> +       struct can327 *elm = (struct can327 *)tty->disc_data;
> +       size_t first_new_char_idx;
> +
> +       if (elm->uart_side_failure)
> +               return;
> +
> +       spin_lock_bh(&elm->lock);
> +
> +       /* Store old rxfill, so elm327_parse_rxbuf() will have
> +        * the option of skipping already checked characters.
> +        */
> +       first_new_char_idx = elm->rxfill;
> +
> +       while (count-- && elm->rxfill < ELM327_SIZE_RXBUF) {
> +               if (fp && *fp++) {
> +                       netdev_err(elm->dev, "Error in received character stream. Check your wiring.");
> +
> +                       elm327_uart_side_failure(elm);
> +
> +                       spin_unlock_bh(&elm->lock);
> +                       return;
> +               }
> +
> +               /* Ignore NUL characters, which the PIC microcontroller may
> +                * inadvertently insert due to a known hardware bug.
> +                * See ELM327 documentation, which refers to a Microchip PIC
> +                * bug description.
> +                */
> +               if (*cp) {
> +                       /* Check for stray characters on the UART line.
> +                        * Likely caused by bad hardware.
> +                        */
> +                       if (!can327_is_valid_rx_char(*cp)) {
> +                               netdev_err(elm->dev,
> +                                          "Received illegal character %02x.\n",
> +                                          *cp);
> +                               elm327_uart_side_failure(elm);
> +
> +                               spin_unlock_bh(&elm->lock);
> +                               return;
> +                       }
> +
> +                       elm->rxbuf[elm->rxfill++] = *cp;
> +               }
> +
> +               cp++;
> +       }
> +
> +       if (count >= 0) {
> +               netdev_err(elm->dev, "Receive buffer overflowed. Bad chip or wiring?");
> +
> +               elm327_uart_side_failure(elm);
> +
> +               spin_unlock_bh(&elm->lock);
> +               return;
> +       }
> +
> +       elm327_parse_rxbuf(elm, first_new_char_idx);
> +       spin_unlock_bh(&elm->lock);
> +}
> +
> +/* Write out remaining transmit buffer.
> + * Scheduled when TTY is writable.
> + */
> +static void can327_ldisc_tx_worker(struct work_struct *work)
> +{
> +       struct can327 *elm = container_of(work, struct can327, tx_work);
> +       ssize_t written;
> +
> +       if (elm->uart_side_failure)
> +               return;
> +
> +       spin_lock_bh(&elm->lock);
> +
> +       if (elm->txleft) {
> +               written = elm->tty->ops->write(elm->tty, elm->txhead, elm->txleft);
> +               if (written < 0) {
> +                       netdev_err(elm->dev,
> +                                  "Failed to write to tty %s.\n",
> +                                  elm->tty->name);
> +                       elm327_uart_side_failure(elm);
> +
> +                       spin_unlock_bh(&elm->lock);
> +                       return;
> +               }
> +
> +               elm->txleft -= written;
> +               elm->txhead += written;
> +       }
> +
> +       if (!elm->txleft)
> +               clear_bit(TTY_DO_WRITE_WAKEUP, &elm->tty->flags);
> +
> +       spin_unlock_bh(&elm->lock);
> +}
> +
> +/* Called by the driver when there's room for more data. */
> +static void can327_ldisc_tx_wakeup(struct tty_struct *tty)
> +{
> +       struct can327 *elm = (struct can327 *)tty->disc_data;
> +
> +       schedule_work(&elm->tx_work);
> +}
> +
> +/* ELM327 can only handle bitrates that are integer divisors of 500 kHz,
> + * or 7/8 of that. Divisors are 1 to 64.
> + * Currently we don't implement support for 7/8 rates.
> + */
> +static const u32 can327_bitrate_const[64] = {

Nitpick but I preffer not to specify the size and let the compiler do it for me:
static const u32 can327_bitrate_const[] = {

If you are worried about the size, you can always do:
static_assert(sizeof(can327_bitrate_const) == 64);

> +        7812,  7936,  8064,  8196,  8333,  8474,  8620,  8771,
> +        8928,  9090,  9259,  9433,  9615,  9803, 10000, 10204,
> +       10416, 10638, 10869, 11111, 11363, 11627, 11904, 12195,
> +       12500, 12820, 13157, 13513, 13888, 14285, 14705, 15151,
> +       15625, 16129, 16666, 17241, 17857, 18518, 19230, 20000,
> +       20833, 21739, 22727, 23809, 25000, 26315, 27777, 29411,
> +       31250, 33333, 35714, 38461, 41666, 45454, 50000, 55555,
> +       62500, 71428, 83333, 100000, 125000, 166666, 250000, 500000
> +};
> +
> +/* Dummy needed to use bitrate_const */
> +static int can327_do_set_bittiming(struct net_device *netdev)
> +{
> +       return 0;
> +}

Is this dummy function really needed? If think that access to
can_priv::do_set_bittiming is always garded. Setting it to NULL should
be OK. c.f.:
https://elixir.bootlin.com/linux/latest/source/drivers/net/can/dev/netlink.c#L198

> +static int can327_ldisc_open(struct tty_struct *tty)
> +{
> +       struct net_device *dev;
> +       struct can327 *elm;
> +       int err;
> +
> +       if (!capable(CAP_NET_ADMIN))
> +               return -EPERM;
> +
> +       if (!tty->ops->write)
> +               return -EOPNOTSUPP;
> +
> +       dev = alloc_candev(sizeof(struct can327), 0);
> +       if (!dev)
> +               return -ENFILE;
> +       elm = netdev_priv(dev);
> +
> +       /* Configure TTY interface */
> +       tty->receive_room = 65536; /* We don't flow control */
> +       spin_lock_init(&elm->lock);
> +       INIT_WORK(&elm->tx_work, can327_ldisc_tx_worker);
> +
> +       /* Configure CAN metadata */
> +       elm->can.bitrate_const = can327_bitrate_const;
> +       elm->can.bitrate_const_cnt = ARRAY_SIZE(can327_bitrate_const);
> +       elm->can.do_set_bittiming = can327_do_set_bittiming;
> +       elm->can.ctrlmode_supported = CAN_CTRLMODE_LISTENONLY;
> +
> +       /* Configure netdev interface */
> +       elm->dev = dev;
> +       dev->netdev_ops = &can327_netdev_ops;
> +
> +       /* Mark ldisc channel as alive */
> +       elm->tty = tty;
> +       tty->disc_data = elm;
> +
> +       devm_can_led_init(elm->dev);
> +
> +       /* Let 'er rip */
> +       err = register_candev(elm->dev);
> +       if (err) {
> +               free_candev(elm->dev);
> +               return err;
> +       }
> +
> +       netdev_info(elm->dev, "can327 on %s.\n", tty->name);
> +
> +       return 0;
> +}
> +
> +/* Close down a can327 channel.
> + * This means flushing out any pending queues, and then returning.
> + * This call is serialized against other ldisc functions:
> + * Once this is called, no other ldisc function of ours is entered.
> + *
> + * We also use this function for a hangup event.
> + */
> +static void can327_ldisc_close(struct tty_struct *tty)
> +{
> +       struct can327 *elm = (struct can327 *)tty->disc_data;
> +
> +       /* unregister_netdev() calls .ndo_stop() so we don't have to.
> +        * Our .ndo_stop() also flushes the TTY write wakeup handler,
> +        * so we can safely set elm->tty = NULL after this.
> +        */
> +       unregister_candev(elm->dev);
> +
> +       /* Mark channel as dead */
> +       spin_lock_bh(&elm->lock);
> +       tty->disc_data = NULL;
> +       elm->tty = NULL;
> +       spin_unlock_bh(&elm->lock);
> +
> +       netdev_info(elm->dev, "can327 off %s.\n", tty->name);
> +
> +       free_candev(elm->dev);
> +}
> +
> +static int can327_ldisc_ioctl(struct tty_struct *tty,
> +                             unsigned int cmd, unsigned long arg)
> +{
> +       struct can327 *elm = (struct can327 *)tty->disc_data;
> +       unsigned int tmp;
> +
> +       switch (cmd) {
> +       case SIOCGIFNAME:
> +               tmp = strnlen(elm->dev->name, IFNAMSIZ - 1) + 1;
> +               if (copy_to_user((void __user *)arg, elm->dev->name, tmp))
> +                       return -EFAULT;
> +               return 0;
> +
> +       case SIOCSIFHWADDR:
> +               return -EINVAL;
> +
> +       default:
> +               return tty_mode_ioctl(tty, cmd, arg);
> +       }
> +}
> +
> +static struct tty_ldisc_ops can327_ldisc = {
> +       .owner          = THIS_MODULE,
> +       .name           = "can327",
> +       .num            = N_CAN327,
> +       .receive_buf    = can327_ldisc_rx,
> +       .write_wakeup   = can327_ldisc_tx_wakeup,
> +       .open           = can327_ldisc_open,
> +       .close          = can327_ldisc_close,
> +       .ioctl          = can327_ldisc_ioctl,
> +};
> +
> +static int __init can327_init(void)
> +{
> +       int status;
> +
> +       status = tty_register_ldisc(&can327_ldisc);
> +       if (status)
> +               pr_err("Can't register line discipline\n");
> +
> +       return status;
> +}
> +
> +static void __exit can327_exit(void)
> +{
> +       /* This will only be called when all channels have been closed by
> +        * userspace - tty_ldisc.c takes care of the module's refcount.
> +        */
> +       tty_unregister_ldisc(&can327_ldisc);
> +}
> +
> +module_init(can327_init);
> +module_exit(can327_exit);
> +
> +MODULE_ALIAS_LDISC(N_CAN327);
> +MODULE_DESCRIPTION("ELM327 based CAN interface");
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Max Staudt <max@enpas.org>");
> diff --git a/include/uapi/linux/tty.h b/include/uapi/linux/tty.h
> index 9d0f06bfbac3..68aeae2addec 100644
> --- a/include/uapi/linux/tty.h
> +++ b/include/uapi/linux/tty.h
> @@ -38,8 +38,9 @@
>  #define N_NULL         27      /* Null ldisc used for error handling */
>  #define N_MCTP         28      /* MCTP-over-serial */
>  #define N_DEVELOPMENT  29      /* Manual out-of-tree testing */
> +#define N_CAN327       30      /* ELM327 based OBD-II interfaces */
>
>  /* Always the newest line discipline + 1 */
> -#define NR_LDISCS      30
> +#define NR_LDISCS      31
>
>  #endif /* _UAPI_LINUX_TTY_H */
> --
> 2.30.2
>