[alsa-devel] [PATCH - dmix v3 0/1] pcm: dmix: Align slave_hw_ptr to slave period boundary

[Timo Wischer]

On 10/30/18 14:21, Takashi Iwai wrote:
 > Yes, and is this correct?  Suppose you start a stream at the position
 > one sample before the next period (psize * N + (psize-1)). Then
 > slave_hw_ptr is psize * N.  At the next moment, the dpcm->hw.ptr
 > reaches to psize * (N+1), and snd_pcm_dmix_sync_ptr() gets called.
 > Then slave_hw_ptr will be updated to psize * (N+1) although only one
 > sample has been processed?

I have created a spreadsheet to simulate the behavior of dmix (see 
attachment). I used this sheet to test different corner cases with 
different implementations.
Column B-D describes 3 corner cases with the original implementation. 
Column E-H describes the corner cases with patch v2 applied and column 
I-L describes the corner cases with patch v3.
With patch v3 I was not able to find a corner case which would fail. All 
our internal audio test are also passing fine.

The corner cases are described in the form
N*period+period-1
(N+1)period+1
(N+2)period+1

The first line describes the value of dmix->spcm->hw.ptr when 
snd_pcm_dmix_start() will be called.
The second line describes the hw_ptr when poll() returns for the first 
time and the third line describes the hw_ptr when poll() returns for the 
second time.

In column B is the issue case described when the patch is not applied. 
snd_pcm_avail() returns 2 frames only (B27) but the buffer only contains 
3 frames (B18).
Therefore up to 5 frames are available (a buffer size of 8 frames is 
choosen).

The issue case of patch v2 is shown in column H. snd_pcm_avail() returns 
7 frames (H46) but the buffer contains still 3 frames (H37). Therefore 
there are only 5 frames free to overwrite.

Do you see any corner cases which I missed or any other drawbacks?

Best regards and thanks for your time

Timo
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