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

Greg KH
2024-02-06T14:50:24Z

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?

thanks,

greg k-h