linux-kernel - Re: [PATCH v3 1/4] platform/x86: intel_scu_ipc: Check status after timeout in busy

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Message-ID: <ZP+D4hlUXhAboFBJ@smile.fi.intel.com>
Date:   Tue, 12 Sep 2023 00:17:22 +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 v3 1/4] platform/x86: intel_scu_ipc: Check status after
 timeout in busy_loop()

On Mon, Sep 11, 2023 at 12:39:33PM -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); // status = IPC_STATUS_BUSY
>   <long time scheduled away>
>   if (!(status & IPC_STATUS_BUSY))
> 
> If this happens, then the status bit could change while the task is
> scheduled away and this function would never read the status again after
> timing out. Instead, the function will return -ETIMEDOUT when it's
> possible that scheduling didn't work out and the status bit was cleared.
> Bit polling code should always check the bit being polled one more time
> after the timeout in case this happens.
> 
> Fix this by reading the status once more after the while loop breaks.
> The read_poll_timeout() macro implements all of this, and it is
> shorter, so use that macro here to consolidate code and fix this.
> 
> There were some concerns with using read_poll_timeout() because it uses
> timekeeping, and timekeeping isn't running early on or during the late
> stages of system suspend or early stages of system resume, but an audit
> of the code concluded that this code isn't called during those times so
> it is safe to use the macro.

...

> +	err = read_poll_timeout(ipc_read_status, status, !(status & IPC_STATUS_BUSY),
> +				100, jiffies_to_usecs(IPC_TIMEOUT), false, scu);

Since "false" you probably can utilize readx_poll_timeout().

> +	if (err)
> +		return err;
>  
-- 
With Best Regards,
Andy Shevchenko