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

    thanks,

    greg k-h

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