Re: [PATCH v5 11/32] sound: usb: card: Introduce USB SND platform op callbacks

[Wesley Cheng <quic_wcheng@xxxxxxxxxxx>]

Hi Takashi,

On 9/7/2023 8:36 AM, Takashi Iwai wrote:
> On Tue, 29 Aug 2023 23:06:36 +0200,
> 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 | 45 +++++++++++++++++++++++++++++++++++++++++++++
> >   sound/usb/card.h |  9 +++++++++
> >   2 files changed, 54 insertions(+)
> >
> > diff --git a/sound/usb/card.c b/sound/usb/card.c
> > index 1b2edc0fd2e9..067a1e82f4bf 100644
> > --- a/sound/usb/card.c
> > +++ b/sound/usb/card.c
> > @@ -118,6 +118,34 @@ 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;
> > +
> > +int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops)
> > +{
> > +	int ret;
> > +
> > +	mutex_lock(&register_mutex);
> > +	if (platform_ops) {
> > +		ret = -EEXIST;
> > +		goto out;
> > +	}
> > +
> > +	platform_ops = ops;
> > +out:
> > +	mutex_unlock(&register_mutex);
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(snd_usb_register_platform_ops);
>
> For adding this kind of API, please give the proper comment.
> Especially this API is special and need a caution, to mention that it
> can be used only for a single instance.
>
> Also, it should be mentioned that all callbacks are exclusive under
> the global register_mutex.

Thanks for taking the time to review.  Sure, I'll add some comments in 
these new APIs to document what they are used for and how they are 
protected and limited.

> > @@ -910,7 +938,11 @@ static int usb_audio_probe(struct usb_interface *intf,
> >   	chip->num_interfaces++;
> >   	usb_set_intfdata(intf, chip);
> >   	atomic_dec(&chip->active);
> > +
> > +	if (platform_ops && platform_ops->connect_cb)
> > +		platform_ops->connect_cb(chip);
> >   	mutex_unlock(&register_mutex);
>
> One uncertain thing is the argument for connect_cb and disconnect_cb.
> Those take snd_usb_audio object, but the callback gets called per
> interface at each probe and disconnect.  How does the callee handle
> multiple calls?

I guess it should depend on how the platform driver wants to handle it? 
 I haven't run into a device with multiple UAC interfaces before, so 
I'll need to mimic this configuration on a device, so I can see how it 
exposes itself.

Will investigate this a bit more on my end and come back with my findings.

> Last but not least, the patch subject should be with "ALSA:" prefix,
> and in this case, at best "ALSA: usb-audio: xxx".

Got it, thanks!

Thanks
Wesley Cheng