[alsa-devel] [PATCH] snd-usb-audio with multiple audio functions

[Takashi Iwai]

At Mon, 6 Jun 2011 10:45:49 +0200,
Daniel Mack wrote:
> 
> Hi Rolf,
> 
> On Mon, Jun 6, 2011 at 8:49 AM, Rolf Meeser <rolfm_9dq at yahoo.de> wrote:
> > Hello,
> >
> > This patch fixes the currently broken support for multiple audio functions
> > (AF) in snd-usb-audio.
> > The patch is against the stable 2.6.39.1 kernel.
> >
> > It addresses three issues:
> > 1. Controls of the second and any further AF are currently not accessibly
> > (controls of the first AF are duplicated!)
> > 2. Multiple AF's are currently incorrectly combined into one sound card
> > index.
> > 3. Module usage count of snd-usb-audio does not drop to zero after
> > disconnecting a multiple AF device.
> >
> > All three issues are fixed by assigning a new card index for each AF in a
> > USB device.
> 
> I assume you have such a device? Which model is that?
> 
> > Rationale:
> >
> > - A USB audio function (AF) consists of a control interface and zero or more
> > streaming interfaces. USB devices can have multiple AF, where the individual
> > AF are completely independent of each other. In particular, it cannot be
> > assumed that the controls of one AF can be manipulated through the control
> > interface of another (the first) AF.
> >
> > - Older kernels (up to 2.6.32? at least before the major rewrite of USB
> > audio code) supported multiple AF's. However, they combined the streams into
> > one sound card: For instance hw:1,0 for the stream interface of the first
> > AF, and hw:1,1 for the stream interface of the second AF.
> > I believe this is incorrect, as different AF's must be treated as an
> > independent entity. Each AF must get its own card index, for the example
> > this would be hw:1,0 and hw:2,0. This is also the way THE OTHER OS
> > implements it.
> >
> > - Newer kernels have broken multiple AF support. In a dual AF setup these
> > kernels detect the second AF, but they duplicate the controls of the first
> > AF, and ignore the controls of the second one!
> >
> > - The patch also fixes a problem with module usage: When you unplug a
> > multiple AF device, the snd-usb-audio module usage count currently stays at
> > 1, while you could expect it to go to zero.
> >
> > This is the first time I'm sending a patch to this list. Please be patient
> > with me if I did something in a wrong way :-)
> 
> The patch itself looks reasonable to be, and I doubt it would break
> any existing functions.

Yeah, I think so, too.

> However, please have a look at Documentation/SubmittingPatches. In
> particular, you need to have a comprehensive commit log (which can be
> taken from your explanations in this email) and your "Signed-off-by:"
> line above the actual patch. Would you care to resend?

FYI, about sign-off (and in general about the kernel patches), refer
to $LINUX/Documentation/SubmittingPatches document.


thanks,

Takashi