[alsa-devel] [PATCH] ALSA: usb: Work around CM6631 sample rate change bug

[Takashi Iwai]

At Sun, 3 Mar 2013 20:29:22 +0100,
Torstein Hegge wrote:
> 
> On Sat, Mar 02, 2013 at 02:12:33PM +0100, Daniel Mack wrote:
> > On 02.03.2013 14:04, Torstein Hegge wrote:
> > > The C-Media CM6631 USB-to-S/PDIF receiver doesn't respond to changes in
> > > sampling frequency while the interface is active.
> > > 
> > > Reset the interface after setting the sampling frequency, to ensure that the
> > > sample rate set by snd_usb_init_sample_rate() is used. Otherwise, the device
> > > will try to use the sample rate of the previous file, causing distorted sound
> > > on sample rate changes.
> > > 
> > > Signed-off-by: Torstein Hegge <hegge at resisty.net>
> > 
> > Acked-by: Daniel Mack <zonque at gmail.com>
> 
> While this does fix the sample rate change problem, it introduces a new
> problem. With at least one media player application (xbmc) this patch
> causes the usb sound card to not output sound when starting a video
> file. Skipping forward or backward in the video stream restores audio
> output.
> 
> I haven't yet been able to look into why this happens, or why xbmc
> behaves different from mplayer and vlc.

If the reset of interface is needed only when *changing* SRC, we can
check the current rate against the previous rate and skip the reset?

The difference of xbmc and mplayer/vlc may come from the different
initialization procedure.  Apps can call hw_params or prepare multiple
times, for example, before actually starting the stream.


Takashi

> 
> Torstein
> 
> > > ---
> > >  sound/usb/pcm.c |   10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
> > > index d82e378..01978a6 100644
> > > --- a/sound/usb/pcm.c
> > > +++ b/sound/usb/pcm.c
> > > @@ -710,6 +710,16 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
> > >  		subs->need_setup_ep = false;
> > >  	}
> > >  
> > > +	/* Some devices doesn't respond to sample rate changes while the
> > > +	 * interface is active. */
> > > +	switch (subs->stream->chip->usb_id) {
> > > +	case USB_ID(0x0d8c, 0x0304): /* C-Media - Schiit USB Interface */
> > > +	case USB_ID(0x0d8c, 0x0309): /* C-Media CM6631 */
> > > +		usb_set_interface(subs->dev, subs->cur_audiofmt->iface,
> > > +					subs->cur_audiofmt->altset_idx);
> > > +		break;
> > > +	}
> > > +
> > >  	/* some unit conversions in runtime */
> > >  	subs->data_endpoint->maxframesize =
> > >  		bytes_to_frames(runtime, subs->data_endpoint->maxpacksize);
> > > 
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