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Subject: Re: [PATCH v7 1/4] can: m_can: Create a m_can platform framework
To:     Dan Murphy <dmurphy@ti.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>
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From:   Wolfgang Grandegger <wg@grandegger.com>
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Message-ID: <1f6f77c6-dcf2-6212-5d4e-1eb699e603f1@grandegger.com>
Date:   Fri, 8 Mar 2019 18:08:36 +0100
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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?

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

I don't think so.

Sorry for confusion.

Wolfgang.