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Date:   Thu, 24 Mar 2022 09:33:40 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Oh Eomji <eomji.oh@samsung.com>
Cc:     Jaroslav Kysela <perex@perex.cz>, Takashi Iwai <tiwai@suse.com>,
        open list <linux-kernel@vger.kernel.org>,
        alsa-devel@alsa-project.org, Leon Romanovsky <leon@kernel.org>,
        Pavel Skripkin <paskripkin@gmail.com>,
        JaeHun Jung <jh0801.jung@samsung.com>
Subject: Re: [PATCH v1 1/3] sound: usb: Add vendor's hooking interface
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On Thu, Mar 24, 2022 at 05:10:42PM +0900, Oh Eomji wrote:
> In mobile, a co-processor can be used with USB audio to improve power
> consumption.  To support this type of hardware, hooks need to be added
> to the USB audio subsystem to be able to call into the hardware when
> needed.
> 
> The main operation of the call-backs are:
>   - Initialize the co-processor by transmitting data when initializing.
>   - Change the co-processor setting value through the interface
>     function.
>   - Configure sampling rate
>   - pcm open/close
>   - other housekeeping
> 
> Known issues:
>   - This only supports one set of callback hooks, meaning that this only
>     works if there is one type of USB controller in the system.  This
>     should be changed to be a per-host-controller interface instead of
>     one global set of callbacks.
> 
> Signed-off-by: JaeHun Jung <jh0801.jung@samsung.com>
> Signed-off-by: Oh Eomji <eomji.oh@samsung.com>
> ---
>  sound/usb/card.c     | 119 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  sound/usb/card.h     |  20 +++++++++
>  sound/usb/usbaudio.h |  45 +++++++++++++++++++
>  3 files changed, 184 insertions(+)
> 
> diff --git a/sound/usb/card.c b/sound/usb/card.c
> index 3769622..bd59311 100644
> --- a/sound/usb/card.c
> +++ b/sound/usb/card.c
> @@ -117,6 +117,117 @@ 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_audio_vendor_ops *usb_vendor_ops;
> +
> +int snd_vendor_set_ops(struct snd_usb_audio_vendor_ops *ops)
> +{
> +	if ((!ops->connect) ||
> +	    (!ops->disconnect) ||
> +	    (!ops->set_interface) ||
> +	    (!ops->set_rate) ||
> +	    (!ops->set_pcm_buf) ||
> +	    (!ops->set_pcm_intf) ||
> +	    (!ops->set_pcm_connection) ||
> +	    (!ops->set_pcm_binterval) ||
> +	    (!ops->usb_add_ctls))
> +		return -EINVAL;
> +
> +	usb_vendor_ops = ops;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(snd_vendor_set_ops);
> +
> +struct snd_usb_audio_vendor_ops *snd_vendor_get_ops(void)
> +{
> +	return usb_vendor_ops;
> +}

This is the function you need to fix up, and add proper reference
counting to, in order to solve your "this breaks with multiple USB
controllers" problem.  So this really should not be all that difficult
of a task.  Why has it taken years to do so?

thanks,

greg k-h