[alsa-devel] no period wakeup support

[Pierre-Louis Bossart]

On 1/30/20 1:46 AM, Jaroslav Kysela wrote:
> Dne 30. 01. 20 v 8:06 Takashi Sakamoto napsal(a):
>> On Wed, Jan 29, 2020 at 05:43:19PM -0600, Pierre-Louis Bossart wrote:
>>>> Nowadays, is this reasonable to consider disabling the period wakeup 
>>>> as default
>>>> instead of expecting period wakeup by default?
>>>
>>> I'd say yes - it's been nearly 10 years since this capability was 
>>> added, and
>>> it's quite common across HDaudio, Chrome, Android plaforms.
>>>
>>> But considering this as a default doesn't mean it's available in 100% 
>>> of the
>>> cases, you still you need to check that
>>>
>>> a) the driver is capable of disabling the period wakeup
>>> b) the driver is capable of providing a precise position outside of 
>>> period
>>> elapsed events (usually marked with the INFO_BATCH capability).
>>>
>>> alsa-lib gives you the means to query both cases.
>>>
>>> Note that you also have the case where you cannot disable interrupts 
>>> but can
>>> still use timer-based solutions (e.g. for USB audio).
>>
>> I suspect this advice.
>>
>> In design of ALSA PCM core, runtime of PCM substream is configured for
>> the mode of no-period-wakeup just in a case that userspace application
>> requests it[1].
>>
>> As long as developers take enough care of compatibility for existent
>> applications, it's better to support period wakeup for each IRQ as a
>> default, then support no-period-wakeup mode as an option.
>>
>>
>> [1] 
>> https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/tree/sound/core/pcm_native.c#n715 
>>
> 
> I agree. We should not break the basic part of the API.

I think you misunderstood my point. I was suggesting an approach similar 
to that of PulseAudio/CRAS/Android, where you first try and use the 
no-period-wakeup, and fallback to the traditional interrupt-based mode 
if it's not possible. The idea is that the no-period-wakeup should work 
now in a majority of the cases, so should be the primary mode 
recommended, not to deprecate or break the period-based solution.