Re: [PATCH] mailbox: pcc: Fix probabilistic command execution timeout

[lihuisong (C)]

On 4/21/2026 6:05 PM, Sudeep Holla wrote:
> On Fri, Apr 17, 2026 at 11:14:29AM +0800, Huisong Li wrote:
> > In some scenarios, PCC command may experience probabilistic timeout.
> > This is primarily caused by the chan_in_use flag being updated after
> > ringing the doorbell, coupled with a lack of proper memory barriers
> > across CPU cores.
> >
> > On fast platforms, a race condition occurs: the platform processing
> > completes and triggers an interrupt before the local CPU sets
> > chan_in_use to true. When the interrupt handler pcc_mbox_irq() runs,
> > it reads chan_in_use as false and incorrectly ignores the interrupt.
> >
> > This patch fixes the race by:
> > 1. Moving the chan_in_use update before ringing the doorbell.
> > 2. Using smp_store_release() to ensure the flag update is visible
> >     to other cores before subsequent hardware or software actions
> >     are triggered.
> > 3. Using smp_load_acquire() in the interrupt handler to ensure the
> >     latest flag value is read before deciding to skip the interrupt.
> Are you seeing the issue on real platforms or you are just reviewing the
> code. I would like to test it on the platform I use but I don't have it
> handy, so may take some time.
Yeah, this is a real issue on my platform. It is probabilistic.
And the problem is gone after this modification.

Gemini AI help me to analyze and suggest that I fix it as this patch done.

> I have added Robbie King who also has helped in testing this PCC driver
> in the past.
Thanks. Please help review this patch.
>