Re: [PATCH v3 09/28] sound: usb: card: Introduce USB SND platform op callbacks

[Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>]

On Wed, Mar 08, 2023 at 03:57:32PM -0800, Wesley Cheng wrote:
> Allow for different platforms to be notified on USB SND connect/disconnect
> seqeunces.  This allows for platform USB SND modules to properly initialize
> and populate internal structures with references to the USB SND chip
> device.
> 
> Signed-off-by: Wesley Cheng <quic_wcheng@xxxxxxxxxxx>
> ---
>  sound/usb/card.c | 36 ++++++++++++++++++++++++++++++++++++
>  sound/usb/card.h | 20 ++++++++++++++++++++
>  2 files changed, 56 insertions(+)
> 
> diff --git a/sound/usb/card.c b/sound/usb/card.c
> index 26268ffb8274..9bcbaa0c0a55 100644
> --- a/sound/usb/card.c
> +++ b/sound/usb/card.c
> @@ -117,6 +117,30 @@ MODULE_PARM_DESC(skip_validation, "Skip unit descriptor validation (default: no)
>  static DEFINE_MUTEX(register_mutex);
>  static struct snd_usb_audio *usb_chip[SNDRV_CARDS];
>  static struct usb_driver usb_audio_driver;
> +static struct snd_usb_platform_ops *platform_ops;

As I've said before, you can not just have one of these.  They need to
be per-bus structure.  Or per-device, something dynamic, not static like
this.

> +int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops)
> +{
> +	if (platform_ops)
> +		return -EEXIST;
> +
> +	mutex_lock(&register_mutex);
> +	platform_ops = ops;
> +	mutex_unlock(&register_mutex);

Your locking is odd for a single pointer, why is it needed at all?

Also you check the pointer before using the lock, which defeats the lock
in the first place.

thanks,

greg k-h