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References: <20220512182921.193462-1-max@enpas.org>
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From:   Vincent Mailhol <vincent.mailhol@gmail.com>
Date:   Fri, 13 May 2022 11:38:31 +0900
Message-ID: <CAMZ6Rq+BwL1NPTLtC5sQAd4z1Kc1TFJPPoW-i+0RZ5dnFaWYiw@mail.gmail.com>
Subject: Re: [PATCH v6] 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,

Nice improvement since v3, thanks! Here are my new comments.

On Fri. 13 May 2022 at 03:29, 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>
> ---
> Many thanks to Greg, Marc, Vincent for reviewing v3!
>
> I've CC'd you hoping you'd check whether you are happy with the changes.
> Please do let me know.
>
>
> Changes in v6:
>  - Renamed 'elmcan' to 'can327'
>  - 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                      | 1152 +++++++++++++++++
>  include/uapi/linux/tty.h                      |    3 +-
>  7 files changed, 1505 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.
> +
> +
> +
> +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.
> +
> +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:
> +
> +    # 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.
> +
> +
> +- 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.
> +
> +
> +
> +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.
> +
> +
> +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``.
> +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
> +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
> +
> +
> +
> +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
>
>  .. 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..bbed01f90161
> --- /dev/null
> +++ b/drivers/net/can/can327.c
> @@ -0,0 +1,1152 @@
> +// 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>
> +
> +#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>
> +#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_to_to_do_bits {

to_to_do? Name looks like a typo. Also, do not see the value on the
_bits suffix. I suggest:

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] ____cacheline_aligned;

Out of curiosity, any rationale for the use of ____cacheline_aligned here?

> +       /* 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 & CAN_EFF_FLAG) ^
> +                   (elm->can_frame_to_send.can_id & CAN_EFF_FLAG)) {

(frame->can_id & CAN_EFF_FLAG) ^ (elm->can_frame_to_send.can_id & CAN_EFF_FLAG)

can be factorized as:

(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);
> +}
> +
> +/* Compare buffer to string length, then compare buffer to fixed string.
> + * This ensures two things:
> + *  - It flags cases where the fixed string is only the start of the
> + *    buffer, rather than exactly all of it.
> + *  - It avoids byte comparisons in case the length doesn't match.
> + *
> + * strncmp() cannot be used here because it accepts the following wrong case:
> + *   strncmp("CAN ER", "CAN ERROR", 6);

What about:
strncmp("CAN ER", "CAN ERROR", 7);
?

> + * This must fail, hence this helper function.
> + */
> +static inline int check_len_then_cmp(const u8 *mem, size_t mem_len, const char *str)
> +{
> +       size_t str_len = strlen(str);
> +
> +       return (mem_len == str_len) && !memcmp(mem, str, str_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 (check_len_then_cmp(elm->rxbuf, len, "UNABLE TO CONNECT")) {

Is this equivalent?
      if (!strncmp(elm->rxbuf, "UNABLE TO CONNECT", len + 1)) {

> +               netdev_err(elm->dev,
> +                          "ELM327 reported UNABLE TO CONNECT. Please check your setup.\n");
> +       } else if (check_len_then_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 (check_len_then_cmp(elm->rxbuf, len, "BUS ERROR")) {
> +               frame->can_id |= CAN_ERR_BUSERROR;
> +       } else if (check_len_then_cmp(elm->rxbuf, len, "CAN ERROR")) {
> +               frame->can_id |= CAN_ERR_PROT;
> +       } else if (check_len_then_cmp(elm->rxbuf, len, "<RX ERROR")) {
> +               frame->can_id |= CAN_ERR_PROT;
> +       } else if (check_len_then_cmp(elm->rxbuf, len, "BUS BUSY")) {
> +               frame->can_id |= CAN_ERR_PROT;
> +               frame->data[2] = CAN_ERR_PROT_OVERLOAD;
> +       } else if (check_len_then_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;
> +       }
> +
> +       /* 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);

You are using many of those lockdep_assert_held(&elm->lock);

I guess you put them for debug purpose and probaly some of those can
be removed (if you see a genuine risk at some places, then OK to keep
as a safety net, but a bit of clean up can be done here, I think).

This comment applies to all use of lockdep_assert_held().

> +
> +       /* 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)));

The second pair of brackets look superficial:
               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 len;
> +       int i;
> +
> +       lockdep_assert_held(&elm->lock);
> +
> +       switch (elm->state) {
> +       case ELM327_STATE_NOTINIT:
> +               elm->rxfill = 0;
> +               break;
> +
> +       case ELM327_STATE_GETDUMMYCHAR:
> +       {

Is this braket need?

> +               /* Wait for 'y' or '>' */
> +               for (i = 0; i < elm->rxfill; i++) {
> +                       if (elm->rxbuf[i] == ELM327_DUMMY_CHAR) {
> +                               elm327_send(elm, "\r", 1);
> +                               elm->state = ELM327_STATE_GETPROMPT;
> +                               i++;
> +                               break;
> +                       } else if (elm327_is_ready_char(elm->rxbuf[i])) {
> +                               elm327_send(elm, ELM327_DUMMY_STRING, 1);
> +                               i++;
> +                               break;
> +                       }
> +               }
> +
> +               elm327_drop_bytes(elm, i);
> +
> +               break;
> +       }

Same.

> +
> +       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. */
> +               for (len = 0;
> +                    (len < elm->rxfill) && (elm->rxbuf[len] != '\r');
> +                    len++) {
> +                       /* empty loop */

Question of taste but would prefer a while look with the len++ in the
body (if you prever to do as above, no need to argue, just keep it
like it is).

> +               }
> +
> +               if (len == ELM327_SIZE_RXBUF) {
> +                       /* Line exceeds buffer. It's probably all 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);
> +                       break;

Can you have just a single break at the end of the case instead of
having it in every branches of the if/else?

> +               } 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);
> +                               break;
> +                       }
> +
> +                       /* No <CR> found - we haven't received a full line yet.
> +                        * Wait for more data.
> +                        */
> +                       break;
> +               }

You just need to introduce a final else block here in order to remove
all those breaks:
              } 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);
> +       }
> +}
> +
> +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;
> +
> +       /* BHs are already disabled, so no spin_lock_bh().
> +        * See Documentation/networking/netdevices.txt
> +        */
> +       spin_lock(&elm->lock);

Do you need to hold the lock here? Isn't it possible to move this line
after the next if so that you do not have to unclock in the error
path?

> +
> +       /* We shouldn't get here after a hardware fault:
> +        * can_bus_off() calls netif_carrier_off()
> +        */
> +       WARN_ON_ONCE(elm->uart_side_failure);
> +
> +       if (!elm->tty ||
> +           elm->uart_side_failure ||
> +           elm->can.ctrlmode & CAN_CTRLMODE_LISTENONLY) {

Can the xmit function get called when CAN_CTRLMODE_LISTENONLY is on?

> +               spin_unlock(&elm->lock);
> +               goto out;
> +       }
> +
> +       netif_stop_queue(dev);
> +
> +       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:

The benefit of the goto label is to factorize code. If you have only
one goto, you might as well prefer to remove the label and do the
kfree_skb inside the if block.

> +       kfree_skb(skb);

Maybe you want to increment dev->stats here?

> +       return NETDEV_TX_OK;
> +}
> +
> +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(char c)
> +{
> +       return (isdigit(c) ||
> +               isupper(c) ||
> +               c == ELM327_DUMMY_CHAR ||
> +               c == ELM327_READY_CHAR ||
> +               c == '<' ||
> +               c == 'a' ||
> +               c == 'b' ||
> +               c == 'v' ||
> +               c == '.' ||
> +               c == '?' ||
> +               c == '\r' ||
> +               c == ' ');

Remark: if this function gets called a lot, you might what so create
an lookup array and:

return c < sizeof(can327_is_valid_rx_char_lookup) &&
can327_is_valid_rx_char_lookup[c];

This should be faster.

> +}
> +
> +/* 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;
> +
> +       spin_lock_bh(&elm->lock);
> +
> +       if (elm->uart_side_failure)
> +               goto out;
> +
> +       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);
> +
> +                       goto out;
> +               }
> +
> +               /* 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 != 0) {

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);

Might make sense to put the netdev_err message inside can327_is_valid_rx_char().

> +                               elm327_uart_side_failure(elm);
> +
> +                               goto out;
> +                       }
> +
> +                       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);
> +
> +               goto out;
> +       }
> +
> +       elm327_parse_rxbuf(elm);
> +
> +out:
> +       spin_unlock_bh(&elm->lock);

If the out label has a single statement, can be better to just repalce
all the goto out with 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);
> +       } else {
> +               spin_unlock_bh(&elm->lock);
> +       }

Remove the else branch and do a common 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] = {
> +        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;
> +}
> +
> +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;

Question: did you try to send/receive DLC greater than 8? (c.f.
CAN_CTRLMODE_CC_LEN8_DLC)

> +
> +       /* 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)
> +               goto out_err;
> +
> +       netdev_info(elm->dev, "can327 on %s.\n", tty->name);
> +
> +       return 0;
> +
> +out_err:

Don't use the goto/lable for a single case.

> +       free_candev(elm->dev);
> +       return err;
> +}
> +
> +/* 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
>