Re: [PATCH v2 2/2] can: m_can: fix missed interrupts with m_can_pci

[Matthias Schiffer <matthias.schiffer@ew.tq-group.com>]

On Mon, 2024-09-23 at 12:17 +0200, Marc Kleine-Budde wrote:
> On 19.09.2024 13:27:28, Matthias Schiffer wrote:
> > The interrupt line of PCI devices is interpreted as edge-triggered,
> > however the interrupt signal of the m_can controller integrated in Intel
> > Elkhart Lake CPUs appears to be generated level-triggered.
> > 
> > Consider the following sequence of events:
> > 
> > - IR register is read, interrupt X is set
> > - A new interrupt Y is triggered in the m_can controller
> > - IR register is written to acknowledge interrupt X. Y remains set in IR
> > 
> > As at no point in this sequence no interrupt flag is set in IR, the
> > m_can interrupt line will never become deasserted, and no edge will ever
> > be observed to trigger another run of the ISR. This was observed to
> > result in the TX queue of the EHL m_can to get stuck under high load,
> > because frames were queued to the hardware in m_can_start_xmit(), but
> > m_can_finish_tx() was never run to account for their successful
> > transmission.
> > 
> > To fix the issue, repeatedly read and acknowledge interrupts at the
> > start of the ISR until no interrupt flags are set, so the next incoming
> > interrupt will also result in an edge on the interrupt line.
> > 
> > Fixes: cab7ffc0324f ("can: m_can: add PCI glue driver for Intel Elkhart Lake")
> > Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> 
> My coworker Lucas pointed me to:
> 
> > https://wiki.linuxfoundation.org/networking/napi#non-level_sensitive_irqs
> 

Thanks. I don't think this is directly applicable here - in our case the lost TX complete interrupts
were the issue, not (only) the RX interrupts.

Matthias


> On the other hand, I would also like to convert the !peripteral part of
> the driver to rx-offload. However, I am still looking for potential
> customers for this task. I have talked to some TI and ST people at LPC,
> maybe they are interested.
> 
> I think let's first fix edge sensitive IRQs, then rework the driver to
> rx-offload.
> 
> regards,
> Marc
> 
>  

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