[pulseaudio-discuss] [PATCH] alsa: Check respect return value of snd_pcm_mmap_commit()

[Peter Meerwald]

Hi,

> > snd_pcm_mmap_commit() actually transfers the memory area prepared by
> snd_pcm_mmap_begin(),
> > it returns the 'count of transferred frames' which should be equal to the
> number of frames
> > returned by snd_pcm_mmap_begin()
> >
> > however, this identify is not checked and the number of frames prepared are
> accounted for,
> > not the number of frames commited -- this is wrong; the ALSA example codes
> bothers to
> > check snd_pcm_mmap_commit()'s returned number of frames
> >
> > this patch outputs a warning and uses the commited number of frames to
> update the bytes
> > read or written, resp.
 
> It's not that easy, I think...
> In case you get fewer frames written, you should retry the mmap_begin + 
> mmap_commit cycle but *without* calling pa_sink_render_into_full, and 
> only with the remaining frames. Right?

ah, yes

I think the question is: does it ever happen?

currently, the code won't notice any glitches and does the wrong thing if 
sframes != frames

the proposed code logs a warning and does the wrong thing if sframes != 
frames

the ALSA example just treats sframes != frames as an error -- probably 
that's the best thing to do (with some logging)?

regards, p.

> > ---
> >   src/modules/alsa/alsa-sink.c   |   11 +++++++++--
> >   src/modules/alsa/alsa-source.c |   17 +++++++++++++----
> >   2 files changed, 22 insertions(+), 6 deletions(-)
> >
> > diff --git a/src/modules/alsa/alsa-sink.c b/src/modules/alsa/alsa-sink.c
> > index e256bbd..fc05232 100644
> > --- a/src/modules/alsa/alsa-sink.c
> > +++ b/src/modules/alsa/alsa-sink.c
> > @@ -639,8 +639,7 @@ static int mmap_write(struct userdata *u, pa_usec_t
> *sleep_usec, bool polled, bo
> >
> >               p = (uint8_t*) areas[0].addr + (offset * u->frame_size);
> >
> > -            written = frames * u->frame_size;
> > -            chunk.memblock = pa_memblock_new_fixed(u->core->mempool, p,
> written, true);
> > +            chunk.memblock = pa_memblock_new_fixed(u->core->mempool, p,
> frames * u->frame_size, true);
> >               chunk.length = pa_memblock_get_length(chunk.memblock);
> >               chunk.index = 0;
> >
> > @@ -660,6 +659,14 @@ static int mmap_write(struct userdata *u, pa_usec_t
> *sleep_usec, bool polled, bo
> >
> >               work_done = true;
> >
> > +            if (PA_UNLIKELY((snd_pcm_uframes_t) sframes != frames)) {
> > +                PA_ONCE_BEGIN {
> > +                    pa_log_warn("ALSA mmap write was set up up for %lu
> frames, but unexpectedly commited %lu frames.\n",
> > +                        (unsigned long) frames, (unsigned long) sframes);
> > +                } PA_ONCE_END;
> > +            }
> > +
> > +            written = sframes * u->frame_size;
> >               u->write_count += written;
> >               u->since_start += written;
> >
> > diff --git a/src/modules/alsa/alsa-source.c b/src/modules/alsa/alsa-source.c
> > index 111c517..36e7585 100644
> > --- a/src/modules/alsa/alsa-source.c
> > +++ b/src/modules/alsa/alsa-source.c
> > @@ -562,6 +562,7 @@ static int mmap_read(struct userdata *u, pa_usec_t
> *sleep_usec, bool polled, boo
> >               const snd_pcm_channel_area_t *areas;
> >               snd_pcm_uframes_t offset, frames;
> >               snd_pcm_sframes_t sframes;
> > +            size_t got;
> >
> >               frames = (snd_pcm_uframes_t) (n_bytes / u->frame_size);
> >   /*             pa_log_debug("%lu frames to read", (unsigned long) frames);
> */
> > @@ -613,16 +614,24 @@ static int mmap_read(struct userdata *u, pa_usec_t
> *sleep_usec, bool polled, boo
> >
> >               work_done = true;
> >
> > -            u->read_count += frames * u->frame_size;
> > +            if (PA_UNLIKELY((snd_pcm_uframes_t) sframes != frames)) {
> > +                PA_ONCE_BEGIN {
> > +                    pa_log_warn("ALSA mmap read was set up up for %lu
> frames, but unexpectedly commited %lu frames.\n",
> > +                        (unsigned long) frames, (unsigned long) sframes);
> > +                } PA_ONCE_END;
> > +            }
> > +
> > +            got = sframes * u->frame_size;
> > +            u->read_count += got;
> >
> >   #ifdef DEBUG_TIMING
> > -            pa_log_debug("Read %lu bytes (of possible %lu bytes)",
> (unsigned long) (frames * u->frame_size), (unsigned long) n_bytes);
> > +            pa_log_debug("Read %lu bytes (of possible %lu bytes)",
> (unsigned long) got, (unsigned long) n_bytes);
> >   #endif
> >
> > -            if ((size_t) frames * u->frame_size >= n_bytes)
> > +            if (got >= n_bytes)
> >                   break;
> >
> > -            n_bytes -= (size_t) frames * u->frame_size;
> > +            n_bytes -= got;
> >           }
> >       }
> >
> >
> 
> 

-- 

Peter Meerwald
+43-664-2444418 (mobile)