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
