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

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Message-ID: <CAE-0n50XCw7ugkoTAUfb5Jrr6-Vh=bvXfTOSeHV_ymyOQfRB2g@mail.gmail.com>
Date:   Tue, 5 Sep 2023 17:27:23 -0500
From:   Stephen Boyd <swboyd@...omium.org>
To:     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,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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()

Quoting Mika Westerberg (2023-08-31 22:50:11)
> 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))
>
> How can the status bit change here as we are the only user and the SCU
> access is serialized by ipclock?

I don't know how the SCU works. I thought that IPC_STATUS_BUSY bit was
cleared by the SCU when it was done processing. With that assumption, I
tried to show that the status is read and then the process schedules
away for a long time and has an outdated view of the busy bit.