Re: [PATCH v13 32/53] ALSA: usb-audio: Check for support for requested audio format

Takashi Iwai
2024-02-06T14:53:46Z

On Tue, 06 Feb 2024 15:50:21 +0100,
Greg KH wrote:
>
> On Tue, Feb 06, 2024 at 02:12:44PM +0100, Takashi Iwai wrote:
> > On Sat, 03 Feb 2024 03:36:24 +0100,
> > Wesley Cheng wrote:
> > >
> > > Allow for checks on a specific USB audio device to see if a requested PCM
> > > format is supported. This is needed for support when playback is
> > > initiated by the ASoC USB backend path.
> > >
> > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
> >
> > Just cosmetic:
> >
> > > +struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
> > > + struct snd_pcm_hw_params *params, int direction)
> > > +{
> > > + struct snd_usb_audio *chip;
> > > + struct snd_usb_substream *subs;
> > > + struct snd_usb_stream *as;
> > > + const struct audioformat *fmt;
> > > +
> > > + /*
> > > + * Register mutex is held when populating and clearing usb_chip
> > > + * array.
> > > + */
> > > + mutex_lock(&register_mutex);
> > > + chip = usb_chip[card_idx];
> > > + if (!chip) {
> > > + mutex_unlock(&register_mutex);
> > > + return NULL;
> > > + }
> > > +
> > > + if (enable[card_idx]) {
> > > + list_for_each_entry(as, &chip->pcm_list, list) {
> > > + subs = &as->substream[direction];
> > > + fmt = snd_usb_find_substream_format(subs, params);
> > > + if (fmt) {
> > > + mutex_unlock(&register_mutex);
> > > + return as;
> > > + }
> > > + }
> > > + }
> > > + mutex_unlock(&register_mutex);
> >
> > I prefer having the single lock/unlock call pair, e.g.
> >
> > struct snd_usb_stream *as, *ret;
> >
> > ret = NULL;
> > mutex_lock(&register_mutex);
> > chip = usb_chip[card_idx];
> > if (chip && enable[card_idx]) {
> > list_for_each_entry(as, &chip->pcm_list, list) {
> > subs = &as->substream[direction];
> > if (snd_usb_find_substream_format(subs, params)) {
> > ret = as;
> > break;
> > }
> > }
> > }
> > mutex_unlock(&register_mutex);
> > return ret;
> > }
> >
> > In this case, we shouldn't reuse "as" for the return value since it
> > can be non-NULL after the loop end.
>
> Why not just use guard(mutex) for this, making it all not an issue?

Heh, it's too new ;)
That should work gracefully, yes.

Takashi