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

6 Feb 2024

      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