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

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Date:   Wed, 6 Sep 2023 13:20:49 -0700
From:   Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Stephen Boyd <swboyd@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
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,
        Prashant Malani <pmalani@...omium.org>
Subject: Re: [PATCH v2 1/3] platform/x86: intel_scu_ipc: Check status after
 timeout in busy_loop()



On 9/6/2023 1:14 PM, Stephen Boyd wrote:
> Quoting Andy Shevchenko (2023-09-06 13:04:54)
>> 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?
> 
> Yes, at the cost of reading the status again when it isn't busy, or
> checking the busy bit after the loop breaks out and reading it once
> again when it is busy. I suppose the compiler would figure that out and
> optimize so that break would simply goto the return statement.
> 
> The code could look like this without a goto.
> 
> 	do {
> 		status = ipc_read_status(scu);
> 		if (!(status & IPC_STATUS_BUSY))
> 			break;
> 	} while (time_before(jiffies, end));
> 
> 	if (status & IPC_STATUS_BUSY)
> 		status = ipc_read_status(scu);

IMO, you can remove the if condition and read again the status in all cases.
It is more readable. But it is up to you.

/* Always read again to double check and get the latest status */
status = ipc_read_status(scu);

> 
> 	if (status & IPC_STATUS_BUSY)
> 		return -ETIMEDOUT;
> 	
> 	return (status & IPC_STATUS_ERR) ? -EIO : 0;

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer