linux-kernel - Re: [PATCH v2 2/2] can: m_can: fix missed interrupts with m_can

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Message-ID: <28314c36f464e1d39f71c0a60997c38fd6775172.camel@ew.tq-group.com>
Date: Mon, 23 Sep 2024 13:06:40 +0200
From: Matthias Schiffer <matthias.schiffer@...tq-group.com>
To: Marc Kleine-Budde <mkl@...gutronix.de>
Cc: Chandrasekar Ramakrishnan <rcsekar@...sung.com>, Vincent Mailhol
 <mailhol.vincent@...adoo.fr>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
 Abeni <pabeni@...hat.com>,  Martin Hundebøll
 <martin@...nix.com>, Markus Schneider-Pargmann <msp@...libre.com>, "Felipe
 Balbi (Intel)" <balbi@...nel.org>, Raymond Tan <raymond.tan@...el.com>,
 Jarkko Nikula <jarkko.nikula@...ux.intel.com>, linux-can@...r.kernel.org, 
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux@...tq-group.com,  lst@...gutronix.de
Subject: Re: [PATCH v2 2/2] can: m_can: fix missed interrupts with m_can_pci

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@...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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