snd_pcm_rewind() weirdness

snd_pcm_rewind() weirdness

[Lennart Poettering]

Hey!

On ALSA 1.0.16 with the HDA driver I am experiencing some weird things
with snd_pcm_rewind() which I think are the result of a kernel bug,
but I am not sure.

Here's what I do to trigger this:

I play some PCM. Eventually I want to rewrite what I already have
written. I call snd_pcm_hwsync() and then snd_pcm_avail_update() to
determine how "empty" the playback buffer is. I subtract that value
from the total buffer size. The resulting value should be something
resembling the number of frames that I might be able to rewind. I then
subtract enough frames to keep away from the playback pointer far
enough (50ms, the playback buffer is 370ms, period is 185ms).

Now I pass that value to snd_pcm_rewind() and it returns something that
is higher than what I passed in. Which I find quite
surprising. I had expected that the returned value could be equal or
smaller than what I passed in, but never higher! 

So, my interpretation of the API was apparently wrong. It looks like
on HDA the snd_pcm_rewind() parameter is apparently rounded *up* to
the next period boundary (which in this case was equivalent to the
*entire* buffer)?

Of course, rewinding that far immediately caused an XRUN. Which is why
I think rounding up in this cases is a bad idea and the API shouldn't
work this way. Rounding down is OK, rounding up is call for trouble.

To tape over this I added some code to immediately call
snd_pcm_forward() with the difference if I noticed that the device
forwarded more than I requested. This value also got rounded up -- to
twice the period size. So with this backward+forward operation I ended
up at a frame index that is farther ahead than where I originally came
from!

So, what's the expected behaviour of snd_pcm_rewind()? Always round
up? What's the rationale behind this?

If this is not a bug but expected behaviour, could we at least add
something for passing the direction to round to?

What I am working on is the "glitch-free" playback model in PulseAudio
(http://0pointer.de/blog/projects/pulse-glitch-free.html). To do this
I am writing blocks smaller than the period size to the device via the
mmap interface all the time. Given that on HDA _rewind() is apparently
not granular to the frame size, is writing via mmap in small blocks
like that OK?  If it's not, how can I learn about this in advance?

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net         ICQ# 11060553
http://0pointer.net/lennart/           GnuPG 0x1A015CC4

Re: snd_pcm_rewind() weirdness

[Jaroslav Kysela]

On Sun, 20 Apr 2008, Lennart Poettering wrote:

> Hey!
> 
> On ALSA 1.0.16 with the HDA driver I am experiencing some weird things
> with snd_pcm_rewind() which I think are the result of a kernel bug,
> but I am not sure.

What alsa-lib plugins are on path? It looks like a rate plugin issue. Post 
also snd_pcm_dump() output. Driver does not do any alignment.

						Jaroslav

-----
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

Re: snd_pcm_rewind() weirdness

[Lennart Poettering]

On Mon, 21.04.08 08:58, Jaroslav Kysela (perex@perex.cz) wrote:

> 
> On Sun, 20 Apr 2008, Lennart Poettering wrote:
> 
> > Hey!
> > 
> > On ALSA 1.0.16 with the HDA driver I am experiencing some weird things
> > with snd_pcm_rewind() which I think are the result of a kernel bug,
> > but I am not sure.
> 
> What alsa-lib plugins are on path? It looks like a rate plugin issue. Post 
> also snd_pcm_dump() output. Driver does not do any alignment.

It's front:0. I don't think the rate plugin is used. 

Device is being opened with
SND_PCM_NO_AUTO_RESAMPLE|SND_PCM_NO_AUTO_CHANNELS|SND_PCM_NO_AUTO_FORMAT
and snd_pcm_hw_params_set_rate_resample(..., 0).

That should get me only softvol, right?

The output of snd_pcm_dump() is here (please ignore the "D:
alsa-util.c" prefix)

<snip>
D: alsa-util.c: Soft volume PCM
D: alsa-util.c: Control: PCM Playback Volume
D: alsa-util.c: min_dB: -51
D: alsa-util.c: max_dB: 0
D: alsa-util.c: resolution: 256
D: alsa-util.c: Its setup is:
D: alsa-util.c:   stream       : PLAYBACK
D: alsa-util.c:   access       : MMAP_INTERLEAVED
D: alsa-util.c:   format       : S16_LE
D: alsa-util.c:   subformat    : STD
D: alsa-util.c:   channels     : 2
D: alsa-util.c:   rate         : 44100
D: alsa-util.c:   exact rate   : 44100 (44100/1)
D: alsa-util.c:   msbits       : 16
D: alsa-util.c:   buffer_size  : 16384
D: alsa-util.c:   period_size  : 8192
D: alsa-util.c:   period_time  : 185759
D: alsa-util.c:   tstamp_mode  : NONE
D: alsa-util.c:   period_step  : 1
D: alsa-util.c:   avail_min    : 31001
D: alsa-util.c:   start_threshold  : -1
D: alsa-util.c:   stop_threshold   : -1
D: alsa-util.c:   silence_threshold: 0
D: alsa-util.c:   silence_size : 0
D: alsa-util.c:   boundary     : 4611686018427387904
D: alsa-util.c: Slave: Hardware PCM card 4 'HDA Intel' device 0 subdevice 0
D: alsa-util.c: Its setup is:
D: alsa-util.c:   stream       : PLAYBACK
D: alsa-util.c:   access       : MMAP_INTERLEAVED
D: alsa-util.c:   format       : S16_LE
D: alsa-util.c:   subformat    : STD
D: alsa-util.c:   channels     : 2
D: alsa-util.c:   rate         : 44100
D: alsa-util.c:   exact rate   : 44100 (44100/1)
D: alsa-util.c:   msbits       : 16
D: alsa-util.c:   buffer_size  : 16384
D: alsa-util.c:   period_size  : 8192
D: alsa-util.c:   period_time  : 185759
D: alsa-util.c:   tstamp_mode  : NONE
D: alsa-util.c:   period_step  : 1
D: alsa-util.c:   avail_min    : 31001
D: alsa-util.c:   start_threshold  : -1
D: alsa-util.c:   stop_threshold   : -1
D: alsa-util.c:   silence_threshold: 0
D: alsa-util.c:   silence_size : 0
D: alsa-util.c:   boundary     : 4611686018427387904
</snip>

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net         ICQ# 11060553
http://0pointer.net/lennart/           GnuPG 0x1A015CC4

Re: snd_pcm_rewind() weirdness

[Lennart Poettering]

On Thu, 24.04.08 02:34, Lennart Poettering (mznyfn@0pointer.de) wrote:

> > > Hey!
> > > 
> > > On ALSA 1.0.16 with the HDA driver I am experiencing some weird things
> > > with snd_pcm_rewind() which I think are the result of a kernel bug,
> > > but I am not sure.
> > 
> > What alsa-lib plugins are on path? It looks like a rate plugin issue. Post 
> > also snd_pcm_dump() output. Driver does not do any alignment.
> 
> It's front:0. I don't think the rate plugin is used. 

OK, I just checked this with using hw:0, i.e. without softvol in the
path. That seems to fix the problem, so I guess softvol is to blame.

BTW: any comment regarding http://mailman.alsa-project.org/pipermail/alsa-devel/2008-April/007354.html?

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net         ICQ# 11060553
http://0pointer.net/lennart/           GnuPG 0x1A015CC4