linux-kernel - Re: [PATCH 3/3] platform/x86: intel_scu

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Message-ID: <CAE-0n508ae9ygTNU+HVk08L2zPy85izLLsyNS-DS7d-LPrffNA@mail.gmail.com>
Date:   Tue, 5 Sep 2023 17:42:26 -0500
From:   Stephen Boyd <swboyd@...omium.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <markgross@...nel.org>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        platform-driver-x86@...r.kernel.org,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Prashant Malani <pmalani@...omium.org>
Subject: Re: [PATCH 3/3] platform/x86: intel_scu_ipc: Fail IPC send if still busy

Quoting Mika Westerberg (2023-08-31 23:06:33)
> On Thu, Aug 31, 2023 at 05:07:26PM +0300, Andy Shevchenko wrote:
> > On Wed, Aug 30, 2023 at 06:14:03PM -0700, Stephen Boyd wrote:
> > > It's possible for interrupts to get significantly delayed to the point
> > > that callers of intel_scu_ipc_dev_command() and friends can call the
> > > function once, hit a timeout, and call it again while the interrupt
> > > still hasn't been processed. This driver will get seriously confused if
> > > the interrupt is finally processed after the second IPC has been sent
> > > with ipc_command(). It won't know which IPC has been completed. This
> > > could be quite disastrous if calling code assumes something has happened
> > > upon return from intel_scu_ipc_dev_simple_command() when it actually
> > > hasn't.
> > >
> > > Let's avoid this scenario by simply returning -EBUSY in this case.
> > > Hopefully higher layers will know to back off or fail gracefully when
> > > this happens. It's all highly unlikely anyway, but it's better to be
> > > correct here as we have no way to know which IPC the status register is
> > > telling us about if we send a second IPC while the previous IPC is still
> > > processing.
> >
> > > +static bool intel_scu_ipc_busy(struct intel_scu_ipc_dev *scu)
> >
> > static int ?
> >
> > > +{
> > > +   u8 status;
> > > +
> > > +   status = ipc_read_status(scu);
> > > +   if (status & IPC_STATUS_BUSY) {
> >
> > > +           dev_err(&scu->dev, "device is busy\n");
> >
> > 1. Wouldn't it exaggerate the logs? Shouldn't be rate limited?
> > 2. OTOH if we return -EBUSY directly from here, do we need this at all?
>
> Agree w/ returning -EBUSY here and dropping the dev_err() (or using
> dev_dbg()).

Ok. I'll change to dev_dbg(). I assume that this should never happen,
but you never know if some calling code will ignore the return -EBUSY
from the previous round and call again while the previous IPC is
processing.