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

[Wesley Cheng]

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@xxxxxxxxxxx>
> > ---
> >   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