[alsa-devel] dsnoop issues
perex at suse.cz
Fri Apr 27 15:00:22 CEST 2007
On Fri, 27 Apr 2007, Takashi Iwai wrote:
> At Fri, 27 Apr 2007 13:27:00 +0100,
> Gordon Miller wrote:
> > Hi
> > All my snd_pcm_readi calls return too few frames. Also when I switch to
> > stereo, the sound is muted. I cannot change this by adjusting period &
> > buffer sizes. dsnoop is enabled by default. The documentation for
> > snd_pcm_readi says that the only way that the frame count can be short
> > is because of a signal or xrun. Well this clearly isn't the case.
> Actually, the documentation is wrong, IMO. The typical behavior of
> read syscall is that it returns a value actually read by that call.
> It doesn't guarantee whether the requested size is filled, and can be
> shorter than requested. As snd_pcm_readi() emulates the read syscall,
> it should behave in that way.
I don't think so completely. For blocking mode (!O_NONBLOCK), all possible
data should be read. Only signal or an error should break this.
The read logic for dsnoop is in snd_pcm_read_areas() in pcm/pcm.c
> The blocking mode means that the function waits until the threshold
> (watermark) condition is fulfilled.
I don't think so. The snd_pcm_read_areas() does only xfer_align alignment
for wrong user input. If less frames are returned, then it is an error
Jaroslav Kysela <perex at suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SUSE Labs
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