On Mon, 4 Jan 2010, ANISH KUMAR wrote:
>
> Dear ALSA Team,
> I have some queries.Please answer them.(previous post please ignore as they couldn't appear because of encoding problem)
> I am getting underrun problem whenever the playback is about to get finished.
> happening in snd_pcm_update_hw_ptr_post function:
> if (avail >= stop_threshold) {
> if (substream->runtime->status->state == SNDRV_PCM_STATE_DRAINING)
> snd_pcm_drain_done(substream);
> else
> xrun(substream); //here it is happening..........
> return -EPIPE; }
>
> 1. As i understand if i dont call drain function
> this will happen.Ismy understanding correct?Because if am calling drain
> function then thisproblem is not happening.If the application doesn't
> call drain functionthen xrun is the only way to stop ALSA or does ALSA
> detect that thereare no more samples to play and automatically stop the
> play(if yes,canyou point out the source code)?
You showed the stop point in above if (avail >= threshold) condition.
> 2. snd_pcm_playback_avail() returns the number of samples that
> stillneeds to be played?Below is the diagram which is my understanding.
No, it's the room for new samples to fill the ring buffer to full.
> Ring buffer (DMA buffer created using preallocated buffer during system initalisation)
> -----------------------------------------------------------------
> | | | | | | |
> -----------------------------------------------------------------
> ^ ^
> | |
> hw_ptr appl_ptr
> After first write call:
> -----------------------------------------------------------------
> | | | | | | |
> -----------------------------------------------------------------
> ^ ^
> | |
> appl_ptr hw_ptr
If hw_ptr > appl_ptr then underrun occurs.
> So my understanding is once wait_for_avail_min returns
> insnd_pcm_lib_write1 function.hw_ptr is increased by some
> amount(periodsize) but appl_ptr will increase only when
> wait_for_avail_minreturns.And difference of
> (hw_ptr+runtime->buffersize-appl_ptr) will give us unplayed samples in
> the case of playback?
No.
> Is this whole ring buffer is filled after the
> snd_pcm_lib_write_transfer function is returned?
Not necessary. It depends how much samples an application feeds in.
> 3.In some rare cases (playback case) i have seen thatwait_for_avail_min
> doesn't return and driver pointer function keeps ongetting called
> causing hw_ptr to increase.This causessnd_pcm_playback_avail() function
> to return a value less than avail_min(as hw_ptr is increasing and
> appl_ptr is stagnant as it is stuck insnd_pcm_lib_write1 function) which
> inturn causes wake-up fromsnd_pcm_update_hw_ptr_post() not to get
> called.When this type ofscenario happens?Is it a normal scenario if
> yes,then how will it willever come out from wait_for_avail_min?
> According to my understanding appl_ptr will increase only when we return
> from wait_for_avail_min?
I'm not sure if you're not mentioning a problem solved in:
http://git.alsa-project.org/?p=alsa-kernel.git;a=commitdiff;h=c91a988dc6551c66418690e36b2a23cdb0255da8
> 4.how does alsa know that it is running short
> of data?As iunderstand that it does not free the memory and data is
> onlyoverwritten.So how does it distinguish between old and new data?
Everything in ALSA PCM is about comparing hw_ptr and appl_ptr. So
basically, the ring buffer size is maximum time for application to fill
and for driver to eat samples. If this time is surpased, wrong things
happen.
> 5.Burst of noise due to stale data problem reported by Jon Smirl and for
> which Mr Jaroslav has suggested this solution
> http://mailman.alsa-project.org/pipermail/alsa-devel/2009-May/017119.html.Is
> it implemented?
Nope. Patches are welcome.
Jaroslav
-----
Jaroslav Kysela <perex@xxxxxxxx>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.
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