Re: Re: should i be able to abort in the middle ofsnd_pcm_mmap_{begin,commit} ?

[Abramo Bagnara <abramo@alsa-project.org>]

Kai Vehmanen wrote:
> 
> On Thu, 21 Feb 2002, Abramo Bagnara wrote:
> 
> >>> if (avail > pcm->buffer_size)
> >> Yes, this condition is faulty. It should be 'avail >= pcm->stop_threshold'.
> > - if avail > buffer_size this means either played garbage (playback) or
> > overwritten samples (capture). The return of an error is perfectly
> > sensible (also because the value returned would not have any sensible
> > meaning).
> 
> Hmm, the mmap semantics of stop_threshold should be similar to the
> read()/write() API.

And this was already true. If you use mmap or read/write is irrelevant
wrt stream stop in XRUN state.

> The functions in alsa-kernel/core/pcm_lib.c seem to take care that
> read/write don't access outside the buffer area. On the other hand it is
> possible to process more bytes than buffer_size with one call.
> 
> Similarly the short pcm mmap example in the ALSA API documentation
> (alsa-lib/src/pcm/pcm.c), can also handle situations where avail_update()
> reports more frames than buffer_size (as can happen after Jaroslav today's
> changes). It always processes period_size at a time.
> 
> It's also true that without the change, stop_threshold didn't work
> properly with mmap access.

??? Why you think that?

> 
> Yes another pov is that Jaroslav changes might cause trouble to some

Not "might", the change already now break all plugins!

> applications (at least jackd will require patching as it sets
> stop_threshold to UINT_MAX - not a big thing in this case). Hmm, could
> this cahnge result in accessing illegal memory areas in some
> situations...?
> 
> > - the insertion of SND_PCM_IOCTL_XRUN is a nonsense, as the application
> > has explicitly asked to not stop the stream when xrun happens.
> 
> It's only called if stop_threshold is set to buffer_size or lower (-> app
> _has_ asked to stop if xrun occurs).

The point is that alsa-lib will stop the stream also when application
has asked to *not* do that.

I call this a _broken_ behaviour, don't you?

That apart perhaps to have a snd_pcm_xrun call (to couple with
snd_pcm_drop and snd_pcm_drain) might be useful (who knows, I don't
believe, but...)

-- 
Abramo Bagnara                       mailto:abramo@alsa-project.org

Opera Unica                          Phone: +39.546.656023
Via Emilia Interna, 140
48014 Castel Bolognese (RA) - Italy

ALSA project               http://www.alsa-project.org
It sounds good!

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