Message-ID: <cea76d80-d85c-2963-2745-4509450e2ea9@grandegger.com>
Date:   Fri, 8 Mar 2019 15:41:39 +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

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).

>>
>> 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.

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