netdev - Re: [PATCH v7 1/4] can: m_can: Create a m

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Date:   Fri, 8 Mar 2019 09:48:32 -0600
From:   Dan Murphy <dmurphy@...com>
To:     Wolfgang Grandegger <wg@...ndegger.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

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?

Again if this code is an issue here I believe this is an issue in the hi3110 and mcp251x

Dan


>>>
>>> In addition in the peripheral context the work queue does not report up to the upper layer the status.
>>> Again the hi3110 and mcp251x drivers are written this way.
>>>
>>> The only issue I see here is that the dropped and invalid check needs to come after the tx_skb check.
>>
>> See above.
> 
> Wolfgang.
> 


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