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

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On 2/6/2024 6:53 AM, Takashi Iwai wrote:
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@xxxxxxxxxxx>

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.


Thanks Greg/Takashi.  That is nifty.

Thanks
Wesley Cheng



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