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

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Date: Tue, 6 Feb 2024 14:50:21 +0000
From: Greg KH <gregkh@...uxfoundation.org>
To: Takashi Iwai <tiwai@...e.de>
Cc: Wesley Cheng <quic_wcheng@...cinc.com>, srinivas.kandagatla@...aro.org,
	mathias.nyman@...el.com, perex@...ex.cz, conor+dt@...nel.org,
	corbet@....net, lgirdwood@...il.com, andersson@...nel.org,
	krzysztof.kozlowski+dt@...aro.org, Thinh.Nguyen@...opsys.com,
	broonie@...nel.org, bgoswami@...cinc.com, tiwai@...e.com,
	robh+dt@...nel.org, konrad.dybcio@...aro.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-sound@...r.kernel.org, linux-usb@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, linux-doc@...r.kernel.org,
	alsa-devel@...a-project.org
Subject: Re: [PATCH v13 32/53] ALSA: usb-audio: Check for support for
 requested audio format

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@...cinc.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