linux-kernel - Re: [PATCH v2 1/3] platform/x86: intel_scu_ipc: Check status after timeout in busy

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Message-ID: <ZPjbZqIQBlIGpoCK@smile.fi.intel.com>
Date:   Wed, 6 Sep 2023 23:04:54 +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 v2 1/3] platform/x86: intel_scu_ipc: Check status after
 timeout in busy_loop()

On Wed, Sep 06, 2023 at 11:09:41AM -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.

...

>  static inline int busy_loop(struct intel_scu_ipc_dev *scu)
>  {
>  	unsigned long end = jiffies + IPC_TIMEOUT;
> +	u32 status;
>  
>  	do {
> -		u32 status;
> -
>  		status = ipc_read_status(scu);
>  		if (!(status & IPC_STATUS_BUSY))

> -			return (status & IPC_STATUS_ERR) ? -EIO : 0;
> +			goto not_busy;

Wouldn't simple 'break' suffice here?

>  		usleep_range(50, 100);
>  	} while (time_before(jiffies, end));
>  
> -	return -ETIMEDOUT;
> +	status = ipc_read_status(scu);
> +	if (status & IPC_STATUS_BUSY)
> +		return -ETIMEDOUT;
> +not_busy:
> +	return (status & IPC_STATUS_ERR) ? -EIO : 0;



>  }

-- 
With Best Regards,
Andy Shevchenko