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

[Takashi Iwai <tiwai@xxxxxxx>]

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.


thanks,

Takashi