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

[Greg KH <gregkh@linuxfoundation.org>]

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