Re: [RFC PATCH 10/14] sound: usb: card: Check for support for requested audio format

[Wesley Cheng <quic_wcheng@quicinc.com>]

Hi Greg,

On 12/24/2022 12:59 AM, Greg KH wrote:
> On Fri, Dec 23, 2022 at 03:31:56PM -0800, 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 for when playback is
>> initiated by the ASoC USB backend path.
>>
>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
>> ---
>>   sound/usb/card.c | 19 +++++++++++++++++++
>>   sound/usb/card.h |  3 +++
>>   2 files changed, 22 insertions(+)
>>
>> diff --git a/sound/usb/card.c b/sound/usb/card.c
>> index 396e5a34e23b..9b8d2ed308c8 100644
>> --- a/sound/usb/card.c
>> +++ b/sound/usb/card.c
>> @@ -133,6 +133,25 @@ int snd_usb_unregister_vendor_ops(void)
>>   }
>>   EXPORT_SYMBOL_GPL(snd_usb_unregister_vendor_ops);
>>   
>> +struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
>> +			struct snd_pcm_hw_params *params, int direction)
>> +{
>> +	struct snd_usb_stream *as;
>> +	struct snd_usb_substream *subs = NULL;
>> +	const struct audioformat *fmt;
>> +
>> +	if (usb_chip[card_idx] && enable[card_idx]) {
>> +		list_for_each_entry(as, &usb_chip[card_idx]->pcm_list, list) {
>> +			subs = &as->substream[direction];
>> +			fmt = find_substream_format(subs, params);
>> +			if (fmt)
>> +				return as;
>> +		}
>> +	}
> 
> Where is the locking here?  How can you walk a list that can be changed
> as you walk it?
> 
> And what about reference counting?  You are returning a pointer to a
> structure, who now "owns" it?  What happens if it is removed from the
> system after you return it?
> 
>> +	return 0;
> 
> Didn't sparse complain about this?  You can't return "0" as a pointer,
> it should be NULL.
> 
> Always run basic tools like sparse on code before submitting it so that
> we don't have to find errors like this.
> 

Got it...I didn't get a chance to run that, but will do it on future 
submissions.  Will also address the locking and pointer reference you 
mentioned.

Thanks
Wesley Cheng