Date: Wed, 4 Oct 2023 11:55:00 +0200
From: Miquel Raynal <miquel.raynal@...tlin.com>
To: Marc Kleine-Budde <mkl@...gutronix.de>
Cc: Wolfgang Grandegger <wg@...ndegger.com>, "David S. Miller"
 <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
 <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>,
 netdev@...r.kernel.org, linux-can@...r.kernel.org, Jérémie
 Dautheribes <jeremie.dautheribes@...tlin.com>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, sylvain.girard@...com,
 pascal.eberhard@...com, stable@...r.kernel.org
Subject: Re: [PATCH v3] can: sja1000: Always restart the Tx queue after an
 overrun

Hi Marc,

mkl@...gutronix.de wrote on Wed, 4 Oct 2023 11:41:08 +0200:

> On 02.10.2023 18:02:06, Miquel Raynal wrote:
> > Upstream commit 717c6ec241b5 ("can: sja1000: Prevent overrun stalls with
> > a soft reset on Renesas SoCs") fixes an issue with Renesas own SJA1000
> > CAN controller reception: the Rx buffer is only 5 messages long, so when
> > the bus loaded (eg. a message every 50us), overrun may easily
> > happen. Upon an overrun situation, due to a possible internal crosstalk
> > situation, the controller enters a frozen state which only can be
> > unlocked with a soft reset (experimentally). The solution was to offload
> > a call to sja1000_start() in a threaded handler. This needs to happen in
> > process context as this operation requires to sleep. sja1000_start()
> > basically enters "reset mode", performs a proper software reset and
> > returns back into "normal mode".
> > 
> > Since this fix was introduced, we no longer observe any stalls in
> > reception. However it was sporadically observed that the transmit path
> > would now freeze. Further investigation blamed the fix mentioned above,
> > and especially the reset operation. Reproducing the reset in a loop
> > helped identifying what could possibly go wrong. The sja1000 is a single
> > Tx queue device, which leverages the netdev helpers to process one Tx
> > message at a time. The logic is: the queue is stopped, the message sent
> > to the transceiver, once properly transmitted the controller sets a
> > status bit which triggers an interrupt, in the interrupt handler the
> > transmission status is checked and the queue woken up. Unfortunately, if
> > an overrun happens, we might perform the soft reset precisely between
> > the transmission of the buffer to the transceiver and the advent of the
> > transmission status bit. We would then stop the transmission operation
> > without re-enabling the queue, leading to all further transmissions to
> > be ignored.
> > 
> > The reset interrupt can only happen while the device is "open", and
> > after a reset we anyway want to resume normal operations, no matter if a
> > packet to transmit got dropped in the process, so we shall wake up the
> > queue. Restarting the device and waking-up the queue is exactly what
> > sja1000_set_mode(CAN_MODE_START) does. In order to be consistent about
> > the queue state, we must acquire a lock both in the reset handler and in
> > the transmit path to ensure serialization of both operations. It turns
> > out, a lock is already held when entering the transmit path, so we can
> > just acquire/release it as well with the regular net helpers inside the
> > threaded interrupt handler and this way we should be safe. As the
> > reset handler might still be called after the transmission of a frame to
> > the transceiver but before it actually gets transmitted, we must ensure
> > we don't leak the skb, so we free it (the behavior is consistent, no
> > matter if there was an skb on the stack or not).
> > 
> > Fixes: 717c6ec241b5 ("can: sja1000: Prevent overrun stalls with a soft reset on Renesas SoCs")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>  
> 
> Have you compile tested this against current net/main?
> 
> |   CC [M]  drivers/net/can/sja1000/sja1000.o
> | drivers/net/can/sja1000/sja1000.c: In function ‘sja1000_reset_interrupt’:
> | drivers/net/can/sja1000/sja1000.c:398:9: error: too few arguments to function ‘can_free_echo_skb’
> |   398 |         can_free_echo_skb(dev, 0);
> |       |         ^~~~~~~~~~~~~~~~~
> | In file included from include/linux/can/dev.h:22,
> |                  from drivers/net/can/sja1000/sja1000.c:62:
> | include/linux/can/skb.h:28:6: note: declared here
> |    28 | void can_free_echo_skb(struct net_device *dev, unsigned int idx,
> |       |      ^~~~~~~~~~~~~~~~~
> |
> 
> This chance is mainline since v5.13-rc1~94^2~297^2~34. I've fixed the
> problem while applying the patch.

I didn't, I fixed that in August and forgot I was on a 5.10 when
submitting, as mainline does not run on the platform I used to test.
Thanks for fixing.

Miquèl

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