linux-kernel - Re: [PATCH 1/3] platform/x86: intel_scu_ipc: Check status after timeouts in busy

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Message-ID: <ZPiFbDWeUV99YE50@smile.fi.intel.com>
Date:   Wed, 6 Sep 2023 16:58:04 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Mika Westerberg <mika.westerberg@...ux.intel.com>,
        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 1/3] platform/x86: intel_scu_ipc: Check status after
 timeouts in busy_loop()

On Tue, Sep 05, 2023 at 05:24:29PM -0500, Stephen Boyd wrote:
> Quoting Andy Shevchenko (2023-08-31 06:53:14)
> > On Wed, Aug 30, 2023 at 06:14:01PM -0700, Stephen Boyd wrote:
> > > It's possible for the polling loop in busy_loop() to get scheduled away
> > > for a long time.
> > >
> > >   status = ipc_read_status(scu);
> > >   <long time scheduled away>
> > >   if (!(status & IPC_STATUS_BUSY))
> > >
> > > If this happens, then the status bit could change and this function
> > > would never test it again after checking the jiffies against the timeout
> > > limit. Polling code should check the condition one more time after the
> > > timeout in case this happens.
> > >
> > > The read_poll_timeout() helper implements this logic, and is shorter, so
> > > simply use that helper here.
> >
> > I don't remember by heart, but on some older Intel hardware this might have
> > been called during early stages where ktime() is not functional yet.
> >
> > Is this still a case here?
> 
> I have no idea if that happens in early stages.

I briefly browsed the current tree and it seems it's not the case.

> What about
> suspend/resume though? I suppose timekeeping could be suspended in that
> case, so we can't really check anything with ktime.

Hmm... SCU itself is running all the time I think. The timekeeping depends on
the platform, but is it really the case? I dunno.

> I can rework this patch to simply recheck the busy bit so that we don't
> have to figure out if the code is called early or from suspend paths.

Yeah, probably we can do this and leave this nice cleanup in place.

-- 
With Best Regards,
Andy Shevchenko