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

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Date: Wed, 7 Feb 2024 16:04:20 -0800
From: Wesley Cheng <quic_wcheng@...cinc.com>
To: Takashi Iwai <tiwai@...e.de>, Greg KH <gregkh@...uxfoundation.org>
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>, <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 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@...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?
> 
> Heh, it's too new ;)
> That should work gracefully, yes.
> 

Thanks Greg/Takashi.  That is nifty.

Thanks
Wesley Cheng