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 16:08:36 -0800
From: Wesley Cheng <quic_wcheng@...cinc.com>
To: Takashi Iwai <tiwai@...e.de>
CC: <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>, <gregkh@...uxfoundation.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

Hi Takashi,

On 2/6/2024 5:12 AM, 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.
> 
> 
Sure will do, thanks for taking a look.

Thanks
Wesley Cheng