Date:   Fri, 8 Mar 2019 19:06:00 +0100
From:   Wolfgang Grandegger <wg@...ndegger.com>
To:     Dan Murphy <dmurphy@...com>, mkl@...gutronix.de,
        davem@...emloft.net
Cc:     linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/4] can: m_can: Create a m_can platform framework



Am 08.03.19 um 18:52 schrieb Dan Murphy:
> 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@...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.

I know,  but for the peripheral devices you have:

  static void m_can_tx_work_queue(struct work_struct *ws)
  {
	struct m_can_priv *priv = container_of(ws, struct m_can_priv,
						tx_work);
	netdev_tx_t ret;

	ret = m_can_tx_handler(priv);
	if (ret == NETDEV_TX_OK)
		priv->tx_skb = NULL;
  }

What will happen with tx_skb if NETDEV_TX_BUSY? It has not been
dropped/freed yet?

Wolfgang.

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