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Subject: Re: [PATCH v7 1/4] can: m_can: Create a m_can platform framework
To:     Wolfgang Grandegger <wg@grandegger.com>, <mkl@pengutronix.de>,
        <davem@davemloft.net>
CC:     <linux-can@vger.kernel.org>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
References: <20190305155220.14037-1-dmurphy@ti.com>
 <5065d6ba-f195-a695-77b1-b837cac1a199@grandegger.com>
 <bbc9db2f-0702-4f24-db64-a41dc0607d2d@ti.com>
 <bf2613ae-67f6-661d-0faa-786308c249a0@grandegger.com>
 <cea76d80-d85c-2963-2745-4509450e2ea9@grandegger.com>
 <6016c8aa-01b6-38d5-0e1f-3a999aae6a13@ti.com>
 <1f6f77c6-dcf2-6212-5d4e-1eb699e603f1@grandegger.com>
 <1e9acd4e-97ad-1d9c-44b6-1b2d1bbe8c0e@ti.com>
 <2026f4ff-31de-3040-0872-5e9d01cc5aa5@grandegger.com>
From:   Dan Murphy <dmurphy@ti.com>
Message-ID: <a2538a05-a03f-06f8-99e3-c7c72287613c@ti.com>
Date:   Fri, 8 Mar 2019 11:52:42 -0600
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On 3/8/19 11:40 AM, Wolfgang Grandegger wrote:
> Hello Dan,
> 
> Am 08.03.19 um 18:25 schrieb Dan Murphy:
>> On 3/8/19 11:08 AM, Wolfgang Grandegger wrote:
>>> Hello,
>>>
>>> Am 08.03.19 um 16:48 schrieb Dan Murphy:
>>>> Wolfgang
>>>>
>>>> On 3/8/19 8:41 AM, Wolfgang Grandegger wrote:
>>>>> Hello Dan,
>>>>>
>>>>> thinking more about it...
>>>>>
>>>>> Am 08.03.19 um 14:29 schrieb Wolfgang Grandegger:
>>>>>> Hello Dan,
>>>>>>
>>>>>> Am 08.03.19 um 13:44 schrieb Dan Murphy:
>>>>>>> Wolfgang
>>>>>>>
>>>>>>> On 3/8/19 4:10 AM, Wolfgang Grandegger wrote:
>>>>>>>> Hallo Dan,
>>>>>>>>
>>>>>>>> Am 05.03.19 um 16:52 schrieb Dan Murphy:
>>>>>>>>> Create a m_can platform framework that peripherial
>>>>>>>>> devices can register to and use common code and register sets.
>>>>>>>>> The peripherial devices may provide read/write and configuration
>>>>>>>>> support of the IP.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Dan Murphy <dmurphy@ti.com>
>>>>>>>>> ---
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> v7 - Fixed remaining new checkpatch issues, removed CSR setting, fixed tx hard
>>>>>>>>> start function to return tx_busy, and renamed device callbacks - https://lore.kernel.org/patchwork/patch/1047220/
>>>>>>>>>
>>>>>>>>> v6 - Squashed platform patch to this patch for bissectablity, fixed coding style
>>>>>>>>> issues, updated Kconfig help, placed mcan reg offsets back into c file, renamed
>>>>>>>>> priv->skb to priv->tx_skb and cleared perp interrupts at ISR start -
>>>>>>>>> Patch 1 comments - https://lore.kernel.org/patchwork/patch/1042446/
>>>>>>>>> Patch 2 comments - https://lore.kernel.org/patchwork/patch/1042442/
>>>>>>>>>
>>>>>>>>>  drivers/net/can/m_can/Kconfig          |  13 +-
>>>>>>>>>  drivers/net/can/m_can/Makefile         |   1 +
>>>>>>>>>  drivers/net/can/m_can/m_can.c          | 700 +++++++++++++------------
>>>>>>>>>  drivers/net/can/m_can/m_can.h          | 110 ++++
>>>>>>>>>  drivers/net/can/m_can/m_can_platform.c | 202 +++++++
>>>>>>>>>  5 files changed, 682 insertions(+), 344 deletions(-)
>>>>>>>>>  create mode 100644 drivers/net/can/m_can/m_can.h
>>>>>>>>>  create mode 100644 drivers/net/can/m_can/m_can_platform.c
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/net/can/m_can/Kconfig b/drivers/net/can/m_can/Kconfig
>>>>>>>>> index 04f20dd39007..f7119fd72df4 100644
>>>>>>>>> --- a/drivers/net/can/m_can/Kconfig
>>>>>>>>> +++ b/drivers/net/can/m_can/Kconfig
>>>>>>>>> @@ -1,5 +1,14 @@
>>>>>>>>>  config CAN_M_CAN
>>>>>>>>> +	tristate "Bosch M_CAN support"
>>>>>>>>> +	---help---
>>>>>>>>> +	  Say Y here if you want support for Bosch M_CAN controller framework.
>>>>>>>>> +	  This is common support for devices that embed the Bosch M_CAN IP.
>>>>>>>>> +
>>>>>>>>> +config CAN_M_CAN_PLATFORM
>>>>>>>>> +	tristate "Bosch M_CAN support for io-mapped devices"
>>>>>>>>>  	depends on HAS_IOMEM
>>>>>>>>> -	tristate "Bosch M_CAN devices"
>>>>>>>>> +	depends on CAN_M_CAN
>>>>>>>>>  	---help---
>>>>>>>>> -	  Say Y here if you want to support for Bosch M_CAN controller.
>>>>>>>>> +	  Say Y here if you want support for IO Mapped Bosch M_CAN controller.
>>>>>>>>> +	  This support is for devices that have the Bosch M_CAN controller
>>>>>>>>> +	  IP embedded into the device and the IP is IO Mapped to the processor.
>>>>>>>>> diff --git a/drivers/net/can/m_can/Makefile b/drivers/net/can/m_can/Makefile
>>>>>>>>> index 8bbd7f24f5be..057bbcdb3c74 100644
>>>>>>>>> --- a/drivers/net/can/m_can/Makefile
>>>>>>>>> +++ b/drivers/net/can/m_can/Makefile
>>>>>>>>> @@ -3,3 +3,4 @@
>>>>>>>>>  #
>>>>>>>>>  
>>>>>>>>>  obj-$(CONFIG_CAN_M_CAN) += m_can.o
>>>>>>>>> +obj-$(CONFIG_CAN_M_CAN_PLATFORM) += m_can_platform.o
>>>>>>>>> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
>>>>>>>>> index 9b449400376b..a60278d94126 100644
>>>>>>>>> --- a/drivers/net/can/m_can/m_can.c
>>>>>>>>> +++ b/drivers/net/can/m_can/m_can.c
>>>>>>>>
>>>>>>>> ... snip...
>>>>>>>>
>>>>>>>>> +static netdev_tx_t m_can_start_xmit(struct sk_buff *skb,
>>>>>>>>> +				    struct net_device *dev)
>>>>>>>>> +{
>>>>>>>>> +	struct m_can_priv *priv = netdev_priv(dev);
>>>>>>>>> +
>>>>>>>>> +	if (can_dropped_invalid_skb(dev, skb))
>>>>>>>>> +		return NETDEV_TX_OK;
>>>>>>>>> +
>>>>>>>>> +	if (priv->is_peripherial) {
>>>>>>>>> +		if (priv->tx_skb) {
>>>>>>>>> +			netdev_err(dev, "hard_xmit called while tx busy\n");
>>>>>>>>> +			return NETDEV_TX_BUSY;
>>>>>>>>> +		}
>>>>>>>>
>>>>>>>> The problem with that approach is, that the upper layer will try to
>>>>>>>> resubmit the current "skb" but not the previous "tx_skb". And the
>>>>>>>> previous "tx_skb" has not been freed yet. I would just drop and free the
>>>>>>>> skb and return NETDEV_TX_OK in m_can_tx_handler() for peripheral devices
>>>>>>>> (like can_dropped_invalid_skb() does).
>>>>>>>>
>>>>>>>
>>>>>>> OK.
>>>>>>>
>>>>>>> So would this also be a bug in the hi3110 and mcp251x drivers (line 521) as well because besides checking tx_length
>>>>>>> this is how these drivers are written.
>>>>>>
>>>>>> This is different. When entering the "start_xmit" routine, the previous
>>>>>> TX is still in progress. It will (hopefully) complete soon. Therefore
>>>>>> returning NETDEV_TX_BUSY is OK. The "start_xmit" routine will be
>>>>>> recalled soon with the same "skb". That scenario should/could also not
>>>>>> happen.
>>>>>
>>>>> In principle, this also applies to the m_can peripheral devices. If
>>>>> tx_skb is not NULL, the TX is still in progress and returning
>>>>> NETDEV_TX_BUSY is just fine.
>>>>>
>>>>>>
>>>>>> In contrast, in "m_can_tx_handler()", the skb could not be handled
>>>>>> because the FIFO is full. The "start_xmit" routine for peripheral
>>>>>> devices for that skb already returned NETDEV_TX_OK. Therefore the only
>>>>>> meaningful action is to drop the skb. Also this error should not happen
>>>>>> and if, something is going really wrong. Therefore I think, a
>>>>>> WARN_ONCE() would be even more appropriate. But that should be a
>>>>>> separate patch.
>>>>>
>>>>> But that's a different issue/error. The tx_skb cannot be processed in
>>>>> "m_can_tx_handler()". Either we drop it or we re-queue it (retry later).
>>>>>
>>>>
>>>> OK I am a bit confused on this.  Are you saying this is not an issue?
>>>> Or are you saying I need to check for tx_len like the other code?
>>>
>>> If you check for tx_skb in the "start_xmit" routine like the hi3110 and
>>> mcp251x, it will work the same way. But only, if the "tx_handler()" has
>>> fully processed the message. It simple means, the TX is still in
>>> progress and will complete soon. But in "m_can_tx_handler()" we return
>>> without handling the message! It will never be sent and freed. Or will
>>> the "m_can_tx_handler()" retry?
>>>
>>
>> I am not seeing where we are not handling the message in the m_can_tx_handler()
> 
> static void m_can_tx_handler(struct m_can_classdev *priv)
> {
> 		...
> 		/* Check if FIFO full */
> 		if (m_can_tx_fifo_full(priv)) {
> 			/* This shouldn't happen */
> 			netif_stop_queue(dev);
> 			netdev_warn(dev,
> 				    "TX queue active although FIFO is full.");
> 			return;
> 		}
> 
> We simply return here. When is the message (tx_skb) processed (sent or freed)?
> What happens with tx_skb?
> 

Are you sure you are looking at the right code?

For patch version v7 I have the following

		/* Check if FIFO full */
		if (m_can_tx_fifo_full(cdev)) {
			/* This shouldn't happen */
			netif_stop_queue(dev);
			netdev_warn(dev,
				    "TX queue active although FIFO is full.");
			return NETDEV_TX_BUSY;
		}

Which is no change from the original source code.

> 
> For the hi3110, we have:
> 
> static void hi3110_tx_work_handler(struct work_struct *ws)
> {
>         struct hi3110_priv *priv = container_of(ws, struct hi3110_priv,
>                                                 tx_work);
>         struct spi_device *spi = priv->spi;
>         struct net_device *net = priv->net;
>         struct can_frame *frame;
> 
>         mutex_lock(&priv->hi3110_lock);
>         if (priv->tx_skb) {
>                 if (priv->can.state == CAN_STATE_BUS_OFF) {
>                         hi3110_clean(net);
>                 } else {
>                         frame = (struct can_frame *)priv->tx_skb->data;
>                         hi3110_hw_tx(spi, frame);
>                         priv->tx_len = 1 + frame->can_dlc;
>                         can_put_echo_skb(priv->tx_skb, net, 0);
>                         priv->tx_skb = NULL;
>                 }
>         }
>         mutex_unlock(&priv->hi3110_lock);
> }
> 
> Either the tx_skb is sent or cleanup (dropped and freed) in case of bus-off.
> Also "hi3110_clean" sets "priv->tx_skb = NULL"! The "tx_len" handles a pending 
> "echo_skb".
> 
>>
>> In the peripheral code the work is queued up.  And the work thread is m_can_tx_work_queue.
>>
>> This in turn calls the m_can_tx_handler and the worker is blocked until return which means the message
>> would have been processed.
>>
>> If there is no issue and the handler returns OK then the skb is set to null.
>> Otherwise the only other time that the skb will not be null is if the FIFO was full.
>>
>> Plus there can only be one work queue at a time so the processing is synchronous.
>> If the upper layer decides to send another packet before the prior one is complete then it will get
>> a TX busy return.
>>
>> IOmapped calls are blocked on return so this is not an issue.  We cannot do it the same way with peripherals due to the
>> atomic context of the request.
> Wolfgang.
> 


-- 
------------------
Dan Murphy