Re: [PATCH v2 2/2] firmware: exynos-acpm: allow use during system shutdown

[Tudor Ambarus <tudor.ambarus@linaro.org>]

On 3/25/25 8:07 AM, Krzysztof Kozlowski wrote:
> On 25/03/2025 09:01, André Draszik wrote:
>> Hi Krzysztof,
>>
>> On Tue, 2025-03-25 at 08:57 +0100, Krzysztof Kozlowski wrote:
>>> On 24/03/2025 16:34, André Draszik wrote:
>>>> +static bool acpm_may_sleep(void)
>>>> +{
>>>> +	return system_state <= SYSTEM_RUNNING ||
>>>> +		(IS_ENABLED(CONFIG_PREEMPT_COUNT) ? preemptible() : !irqs_disabled());
>>>> +}
>>>> +
>>>>  /**
>>>>   * acpm_dequeue_by_polling() - RX dequeue by polling.
>>>>   * @achan:	ACPM channel info.
>>>> @@ -300,7 +314,10 @@ static int acpm_dequeue_by_polling(struct acpm_chan *achan,
>>>>  			return 0;
>>>>  
>>>>  		/* Determined experimentally. */
>>>> -		usleep_range(20, 30);
>>>> +		if (!acpm_may_sleep())
>>>> +			udelay(10);
>>>> +		else
>>>
>>> ... and what do you do if IRQs get disabled exactly in this moment? This
>>> is just racy. You cannot check for a condition and assume it will be
>>> valid for whatever time you want it to be valid.
>>>
>>> What happens if system_state is changed to shutdown in this particular
>>> moment? How did you prevent this from happening?
>>
>> Yes, and that's also what the I2C subsystem is doing, AFAICS, see
>> i2c_in_atomic_xfer_mode() and its use. This is to make a very
>> specific corner case work, similar to I2C which has to deal with
>> the same issue during shutdown.
> 
> But they don't have a choice so they try to do the best to avoid
> sleeping. And it is a subsystem, not a driver, which means their
> patterns are sometimes special. Drivers should not replicate subsystem
> workarounds.
> 
>>
>> Would you have a better suggestion?
> 
> Yes, you have a choice, you can always use udelay. Driver code is
> supposed to be always correct.

Using udelay() is good enough for now. I see that downstream uses a
usleep_range(50, 100) and I'm concerned that we're going to waste lots
of cpu cyles once more and more clients get added.

If there's no concurrency on the ACPM queue mutexes at late system
shutdown, would it work to pass the don't sleep requirement from the
client to ACPM and use udelay only then?

Cheers,
ta