[PATCH 16/31] ALSA: usb-audio: scarlett2: Add Gen 3 mixer support

[Takashi Iwai]

On Tue, 22 Jun 2021 09:58:27 +0200,
Takashi Iwai wrote:
> 
> On Tue, 22 Jun 2021 09:44:54 +0200,
> Geoffrey D. Bennett wrote:
> > 
> > On Tue, Jun 22, 2021 at 09:34:25AM +0200, Takashi Iwai wrote:
> > > On Tue, 22 Jun 2021 09:07:20 +0200,
> > > Vladimir Sadovnikov wrote:
> > > > 
> > > > Hello Takashi!
> > > > 
> > > > Since Focusrite devices are too advanced in settings, the overall
> > > > amount of 256 controls is not enough for these devices (like 18i20).
> > > > I would like also to extend this constant up to 1024 or even more
> > > > since adding support of software configuration of the device also
> > > > can exceed the amount of 512 control elements.
> > > 
> > > This define isn't for the total number of mixer elements.  Instead,
> > > it's just a size of the bitmap table that contains the head of the
> > > linked list for each unit id (in the sense of USB mixer spec).
> > > So the number of mixer elements is unlimited.
> > 
> > Sorry I don't understand what's going on then. Am I calling
> > snd_usb_mixer_add_control() wrong? Because when I called it more than
> > MAX_ID_ELEMS times I got a buffer overflow in mixer->id_elems[] (from
> > memory, I can confirm tonight).
> > 
> > snd_usb_create_mixer() has:
> > 
> >         mixer->id_elems = kcalloc(MAX_ID_ELEMS, sizeof(*mixer->id_elems),
> >                                   GFP_KERNEL);
> > 
> > snd_usb_mixer_add_control() called from mixer_scarlett_gen2.c ends up
> > at snd_usb_mixer_add_list() which does:
> > 
> >         list->next_id_elem = mixer->id_elems[list->id];
> >         mixer->id_elems[list->id] = list;
> > 
> > And list->id was going over MAX_ID_ELEMS.
> 
> Here, list->id is the *USB* mixer unit id, not the ALSA control id or
> whatever the internal index.  The former should be a byte, hence it
> can't be over 256.
> 
> That said, the scarlett2 code calls the function in a wrong way.  It
> has worked casually, so far, just because the core code doesn't use
> the unit id number for significant roles.

... and looking again at the code, actually it does matter.
The USB audio mixer code calls the resume for each mixer element, and
the default resume code (default_mixer_resume()) is applied for a
control with usb_mixer_elem_info.val_type == USB_MIXER_BOOLEAN and
channels == 1.  It seems that scarlett2 has some of them, and the
resume procedure is applied wrongly with an invalid unit id.

We need to define a special mixer value type (e.g. USB_MIXER_VENDOR),
and use this for the all scarlett2 mixer elements, in addition to the
below:

> So, as a quick workaround, simply pass 0 or any fixed number under 256
> to list->id (i.e. elem->head.id in scarlett2_add_new_ctl()).  That's
> all, and the elements are chained in the linked list.


Takashi