Hi Marc,
mkl@pengutronix.de wrote on Wed, 27 Sep 2023 11:33:32 +0200:
On 27.09.2023 11:30:16, Marc Kleine-Budde wrote:
On 22.09.2023 17:47:27, 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. 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).
Can you make use of netif_tx_disable() and netif_wake_queue() in sja1000_reset_interrupt() instead of the lock?
...or netif_tx_lock()/netif_tx_unlock().
As that's also a spinlock behind I guess it would fit. A quick look does not seem to show any specific constraint in using it, so let's go for it.
Thanks, Miquèl