[alsa-devel] [PATCH - dmix v2 1/1] pcm: dmix: Align slave_hw_ptr to slave period boundary

[twischer at de.adit-jv.com]

From: Laxmi Devi <Laxmi.Devi at in.bosch.com>

These changes are required due to the kernel commit 07b7acb51d283d8469696c906b91f1882696a4d4
("ASoC: rsnd: update pointer more accurate")

Issue is that snd_pcm_wait() goes back to waiting because the hw_ptr
is not period aligned. Therefore snd_pcm_wait() will block for a longer
time as required.

With these rcar driver changes the exact position of the dma is returned.
During snd_pcm_start they read hw_ptr as reference, and this hw_ptr
is now not period aligned, and is a little ahead over the period while it
is read. Therefore when the avail is calculated during snd_pcm_wait(),
it is missing the avail_min by a few frames.
Consider the below example:

Considering the application is trying to write 0x120 frames and the
period_size = 0x60, avail_min = 0x120 and buffersize = 0x360 :

rsnd_pointer=0x12c -> dma pointer at the end of second write during
snd_pcm_dmix_start().
Since another 0x120 buffer is available, application writes 0x120 and goes
to snd_pcm_wait().
It is woken up after 3 snd_pcm_period_elapsed() to see rsnd_pointer=0x248.
So hw_ptr = new_slave_hw_ptr - reference_slave_hw_ptr = 0x248 - 0x12c = 0x11c.
It needs 4 more frames to be able to write. And so it goes back to waiting.

But since 3 snd_pcm_period_elapsed(), 3 periods should be available and it
should have been able to write.
If rsnd_pointer during the start was 0x120 which is 3 periods
then 0x248 - 0x120 =  128 it could go on with write.

Signed-off-by: Laxmi Devi <Laxmi.Devi at in.bosch.com>
---

> On 10/29/18 16:54, Takashi Iwai wrote:
> The problem is that aligning the start essentially imposes an
> artificial long latency, and changes the behavior out of sudden.

Now, we are only align the salve_hw_ptr which also solves our delay issue.
But this change should not increase the latency because the write position
is given by slave_appl_ptr.
Do you see any other drawbacks?

Best regards

Timo

diff --git a/src/pcm/pcm_dmix.c b/src/pcm/pcm_dmix.c
index a6a8f3a..eaa0b0f 100644
--- a/src/pcm/pcm_dmix.c
+++ b/src/pcm/pcm_dmix.c
@@ -560,6 +560,8 @@ static int snd_pcm_dmix_hwsync(snd_pcm_t *pcm)
 static void reset_slave_ptr(snd_pcm_t *pcm, snd_pcm_direct_t *dmix)
 {
 	dmix->slave_appl_ptr = dmix->slave_hw_ptr = *dmix->spcm->hw.ptr;
+	dmix->slave_hw_ptr = ((dmix->slave_hw_ptr / dmix->slave_period_size)
+				* dmix->slave_period_size);
 	if (pcm->buffer_size > pcm->period_size * 2)
 		return;
 	/* If we have too litte periods, better to align the start position
-- 
2.7.4