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

[Takashi Iwai <tiwai@suse.de>]

On Wed, 12 Aug 2020 08:53:42 +0200,
Yu-Hsuan Hsu wrote:
> 
> Takashi Iwai <tiwai@suse.de> 於 2020年8月12日 週三 下午2:14寫道：
> >
> > On Wed, 12 Aug 2020 05:09:58 +0200,
> > Yu-Hsuan Hsu wrote:
> > >
> > > Mark Brown <broonie@kernel.org> 於 2020年8月12日 週三 上午1:22寫道：
> > > >
> > > > On Tue, Aug 11, 2020 at 11:54:38AM -0500, Pierre-Louis Bossart wrote:
> > > >
> > > > > > constraint logic needs to know about this DSP limitation - it seems like
> > > > > > none of this is going to change without something new going into the
> > > > > > mix?  We at least need a new question to ask about the DSP firmware I
> > > > > > think.
> > > >
> > > > > I just tested aplay -Dhw: on a Cyan Chromebook with the Ubuntu kernel 5.4,
> > > > > and I see no issues with the 240 sample period. Same with 432, 960, 9600,
> > > > > etc.
> > > >
> > > > > I also tried just for fun what happens with 256 samples, and I don't see any
> > > > > underflows thrown either, so I am wondering what exactly the problem is?
> > > > > Something's not adding up. I would definitively favor multiple of 1ms
> > > > > periods, since it's the only case that was productized, but there's got to
> > > > > me something a side effect of how CRAS programs the hw_params.
> > > >
> > > > Is it something that goes wrong with longer playbacks possibly (eg,
> > > > someone watching a feature film or something)?
> > >
> > > Thanks for testing!
> > >
> > > After doing some experiments, I think I can identify the problem more precisely.
> > > 1. aplay can not reproduce this issue because it writes samples
> > > immediately when there are some space in the buffer. However, you can
> > > add --test-position to see how the delay grows with period size 256.
> > > > aplay -Dhw:1,0 --period-size=256 --buffer-size=480 /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 = 2064, buffer = 512
> > > Suspicious buffer position (2 total): avail = 0, delay = 2064, buffer = 512
> > > Suspicious buffer position (3 total): avail = 0, delay = 2096, buffer = 512
> > > ...
> >
> > Isn't this about the alignment of the buffer size against the period
> > size, not the period size itself?  i.e. in the example above, the
> > buffer size isn't a multiple of period size, and DSP can't handle if
> > the position overlaps the buffer size in a half way.
> >
> > If that's the problem (and it's an oft-seen restriction), the right
> > constraint is
> >   snd_pcm_hw_constraint_integer(runtime, SNDRV_PCM_HW_PARAM_PERIODS);
> >
> >
> > Takashi
> Oh sorry for my typo. The issue happens no matter what buffer size is
> set. Actually, even if I want to set 480, it will change to 512
> automatically.
> Suspicious buffer position (1 total): avail = 0, delay = 2064, buffer
> = 512 <-this one is the buffer size

OK, then it means that the buffer size alignment is already in place.

And this large delay won't happen if you use period size 240?


Takashi

> > > 2. Since many samples are moved to DSP(delay), the measured rate of
> > > the ring-buffer is high. (I measured it by alsa_conformance_test,
> > > which only test the sampling rate in the ring buffer of kernel not
> > > DSP)
> > >
> > > 3. Since CRAS writes samples with a fixed frequency, this behavior
> > > will take all samples from the ring buffer, which is seen as underrun
> > > by CRAS. (It seems that it is not a real underrun because that avail
> > > does not larger than buffer size. Maybe CRAS should also take dalay
> > > into account.)
> > >
> > > 4. In spite of it is not a real underrun, the large delay is still a
> > > big problem. Can we apply the constraint to fix it? Or any better
> > > idea?
> > >
> > > Thanks,
> > > Yu-Hsuan
> > >
>