[alsa-devel] Misusing snd_pcm_avail_update()

Lennart Poettering mznyfn at 0pointer.de
Wed Jan 28 19:30:01 CET 2009

On Fri, 23.01.09 18:56, Clemens Ladisch (clemens at ladisch.de) wrote:

> Takashi Iwai wrote:
> > [...]
> > My main concern is what kernel <-> user API is needed in addition or
> > needed to be changed.
> > 
> > If it's a question how to pass the granularity to user-space, usually
> > it's a constant value, and thus it can be put somewhere in the
> > existing struct, or add a single ioctl. 
> Most PCI devices have 32 bytes; wavetable chips have a constant time
> (5.33 ms, i.e., resampled to 256 framesat 48 kHz).  But the interesting
> cases are where the granularity is dependent on the period size, or
> where the application could choose some arbitrary value (USB).  For
> these cases, it would be very useful to have the granularity as an
> interval in the PCM hardware parameters  (or probably three: bytes/
> frames/time).
> In the case of granularity==period, this allows PulseAudio to detect
> that it has to work with small periods after it has set a small upper
> bound for the granularity.  (This is exactly what the hw_param
> dependencies were designed for.)
> > OTOH, if it has to be implemented as a form of snd_pcm_busy_for(),
> > the kernel needs the compuation like the above.  That's my concern.
> Instead of writing a callback in the USB driver to compute the time
> until the next underrun, I'd rather rip out that fast start code.
> So, no kernel computation is needed.  :-)

While I think it would be good not have this kind of double-buffering
I wonder if this is really future-proof. i.e. can this done with every
driver that uses 'fast starts'? 

> Anyway, regardless of how the API looks, I see two compatibility
> concerns:
> * For many devices (legacy ISA, etc.), we just don't know the correct
>   value.

But it should be possible to pick a safe boundary, shouldn't it?

> * What should alsa-lib do when it runs on an old kernel?  It could
>   return a worst-case estimate (period size), but this would cause PA
>   to use small periods.  Perhaps it would be better to return some error
>   ("don't know").

If we'd do it the the busy_for() API we could simply return
buffer_size - avail_update() - extra_margin. Or simply return
ENOSUPP. That would be fine too.


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

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