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

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Date:   Fri, 8 Mar 2019 14:29:49 +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,

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 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.

> 
> 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.