Re: [PATCH v3 2/2] ASoC: Intel: Add period size constraint on strago board

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

On 8/13/20 3:45 AM, Takashi Iwai wrote:
> On Thu, 13 Aug 2020 10:36:57 +0200,
> Yu-Hsuan Hsu wrote:
>>
>> Lu, Brent <brent.lu@intel.com> 於 2020年8月13日 週四 下午3:55寫道：
>>>
>>>>>>
>>>>>> CRAS calls snd_pcm_hw_params_set_buffer_size_max() to use as large
>>>>>> buffer as possible. So the period size is an arbitrary number in
>>>>>> different platforms. Atom SST platform happens to be 256, and CML
>>>>>> SOF platform is 1056 for example.
>>>>>
>>>>> ok, but earlier in this thread it was mentioned that values such as
>>>>> 432 are not suitable. the statement above seems to mean the period
>>>>> actual value is a "don't care", so I don't quite see why this specific
>>>>> patch2 restricting the value to 240 is necessary. Patch1 is needed for
>>>>> sure,
>>>>> Patch2 is where Takashi and I are not convinced.
>>>>
>>>> I have downloaded the patch1 but it does not work. After applying patch1,
>>>> the default period size changes to 320. However, it also has the same issue
>>>> with period size 320. (It can be verified by aplay.)
>>>
>>> The period_size is related to the audio latency so it's decided by application
>>> according to the use case it's running. That's why there are concerns about
>>> patch 2 and also you cannot find similar constraints in other machine driver.
>> You're right. However, the problem here is the provided period size
>> does not work. Like 256, setting the period size to 320 also makes
>> users have big latency in the DSP ring buffer.
>>
>> localhost ~ # aplay -Dhw:1,0 --period-size=320 --buffer-size=640
>> /dev/zero -d 1 -f dat --test-position
>> Playing raw data '/dev/zero' : Signed 16 bit Little Endian, Rate 48000
>> Hz, Stereo
>> Suspicious buffer position (1 total): avail = 0, delay = 2640, buffer = 640
>> Suspicious buffer position (2 total): avail = 0, delay = 2640, buffer = 640
>> Suspicious buffer position (3 total): avail = 0, delay = 2720, buffer = 640
>> ...
> 
> It means that the delay value returned from the driver is bogus.
> I suppose it comes pcm_delay value calculated in sst_calc_tstamp(),
> but haven't followed the code closely yet.  Maybe checking the debug
> outputs can help to trace what's going wrong.

the problem is really that we add a constraint that the period size be a 
multiple of 1ms, and it's not respected. 320 samples is not a valid 
choice, I don't get how it ends-up being selected? there's a glitch in 
the matrix here.