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

[Takashi Iwai]

On Tue, 06 Nov 2018 13:52:53 +0100,
<twischer at de.adit-jv.com> wrote:
> 
> 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>
> Signed-off-by: Timo Wischer <twischer at de.adit-jv.com>
> ---
> 
> With this patch the slave_hw_ptr will always contain a period align value.

By enforcing that, application won't be able to get the fine-grained
hwptr update any longer, hence it is a clear regression.

That is, we can't apply this as is.  If any, such adjustment has to be
applied only conditionally for certain hardware setup.


thanks,

Takashi