Re: [PATCH 2/3 v2] ALSA: usb-audio: avoid setting of sample rate multiple times on bus

[Jiada Wang]

Hi Sakamoto

On 11/30/2016 02:45 AM, Takashi Sakamoto wrote:
> Hi Jiada,
>
> I don't oppose this patch. Nevertheless, your description is not 
> necessarily correct.
>
> On Nov 30 2016 16:59, Jiada Wang wrote:
> > From: Daniel Girnus <dgirnus@xxxxxxxxxxxxxx>
> >
> > ALSA usually calls the prepare function twice before starting the 
> > playback:
> > 1. On hw_params call from userland and
> > 2. internally when starting the stream.
>
> ALSA PCM core in kernel land doesn't perform like this.
>
> In alsa-lib, 'snd_pcm_hw_params()' calls 
> 'snd_pcm_hw_params_internal()' and 'snd_pcm_prepare()' sequentially.
> http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/pcm/pcm.c;h=cd87bc759ded95953e332b7e8d56b0f2d5b4185d;hb=HEAD#l853 
>
>
> In system call level (e.g. see by strace(1)), this looks like two 
> ioctl(2)s with 'SNDRV_PCM_IOCTL_HW_PARAMS' and 'SNDRV_PCM_IOCTL_PREPARE'.
>
> Well, when applications are written to execute 'snd_pcm_hw_params()' 
> and 'snd_pcm_hw_prepare()' sequentially, additional ioctl(2) with 
> 'SNDRV_PCM_IOCTL_PREPARE' appears. PulseAudio is this kind of 
> application. I indicated the useless in 2014, but it still remains:
> https://lists.freedesktop.org/archives/pulseaudio-discuss/2014-January/019773.html 
>
>
> You have the misunderstanding due to a nature of alsa-lib and tendency 
> of major applications, from my point of view.
Thanks for your indication, so because some of userland applications 
call 'snd_pcm_hw_params()' and
'snd_pcm_hw_prepare()' sequentially, means the second 
'SNDRV_PCM_IOCTL_PREPARE' be called in 'SNDRV_PCM_STATE_PREPARED' state, 
some devices are unable to manage this and stop working.

I will update Changelog in v2 Patchset.

Thanks,
Jiada

> > Some device are not able to manage this and they will stop playback
> > if the sample rate will be configured several times over USB protocol.
> >
> > Signed-off-by: Jens Lorenz <jlorenz@xxxxxxxxxxxxxx>
> > Signed-off-by: Jiada Wang <jiada_wang@xxxxxxxxxx>
> > ---
> >  sound/usb/pcm.c | 21 +++++++++++----------
> >  1 file changed, 11 insertions(+), 10 deletions(-)
> >
> > diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
> > index 44d178e..a522c9a 100644
> > --- a/sound/usb/pcm.c
> > +++ b/sound/usb/pcm.c
> > @@ -806,17 +806,18 @@ static int snd_usb_pcm_prepare(struct 
> > snd_pcm_substream *substream)
> >      if (ret < 0)
> >          goto unlock;
> >
> > -    iface = usb_ifnum_to_if(subs->dev, subs->cur_audiofmt->iface);
> > -    alts = &iface->altsetting[subs->cur_audiofmt->altset_idx];
> > -    ret = snd_usb_init_sample_rate(subs->stream->chip,
> > -                       subs->cur_audiofmt->iface,
> > -                       alts,
> > -                       subs->cur_audiofmt,
> > -                       subs->cur_rate);
> > -    if (ret < 0)
> > -        goto unlock;
> > -
> >      if (subs->need_setup_ep) {
> > +
> > +        iface = usb_ifnum_to_if(subs->dev, subs->cur_audiofmt->iface);
> > +        alts = &iface->altsetting[subs->cur_audiofmt->altset_idx];
> > +        ret = snd_usb_init_sample_rate(subs->stream->chip,
> > +                           subs->cur_audiofmt->iface,
> > +                           alts,
> > +                           subs->cur_audiofmt,
> > +                           subs->cur_rate);
> > +        if (ret < 0)
> > +            goto unlock;
> > +
> >          ret = configure_endpoint(subs);
> >          if (ret < 0)
> >              goto unlock;
>
>
> Regards
>
> Takashi Sakamoto