[alsa-devel] [PATCH 1/5] allow up to 32 PCM devices

[Takashi Iwai]

At Wed, 12 May 2010 16:39:50 +0800,
Wu Fengguang wrote:
> 
> On Wed, May 12, 2010 at 04:03:43PM +0800, Takashi Iwai wrote:
> > At Wed, 12 May 2010 09:29:57 +0200 (CEST),
> > Jaroslav Kysela wrote:
> > > 
> > > On Wed, 12 May 2010, Wu Fengguang wrote:
> > > 
> > > > Reserve 32 minor numbers for PCM playback devices.
> > > >
> > > > The Intel SandyBridge HDMI audio codec provides 3 PCM devices with
> > > > indices 3, 7, 8. Among which the device 8's minor number will be
> > > > overlapped with the first capture device's minor number in the current
> > > > static minor number allocation scheme.
> > > >
> > > > Also increase SNDRV_PCM_DEVICES to make pcm_dev_bits big enough to hold
> > > > the increasing number of PCM devices.
> > > 
> > > I don't agree to have only 4 slots for soundcards in the static minor 
> > > numbering. Maybe the driver should be converted to use subdevices or we 
> > > might drop the static minor number allocation at all (it might have only 
> > > impact for old distros).
> 
> Jaroslav, will there be so many sound cards in one system?

In the old time, yes.  Now we have less and less PCI slots.
In theory, we may have lots of USB audio devices, though :)

Another possible solution would be to change the minor number
assignment to a really dynamic one.  So far, due to legacy /dev/aload
and co, we have some static restriction per card basis.


> > Dropping such a base feature is really no good option.  Better to give
> > simply an error for more than 8 PCMs in such a case, IMO.
> 
> Agreed.
> 
> > I also wonder whether having 4 individual PCMs is a way to go.  We may
> > have PCM substreams, if any.  OTOH, the current IEC958 stream
> > assignment mechanism doesn't consider multiple substreams well, e.g.
> > we have no proper way to match the IEC958 status bits control to a PCM
> > substream.
> 
> Hmm, what if there are two monitors attached, each want to play a
> different music track?

PCM substreams are really independent from each other.  So,
technically seen, substreams are feasible.

> I think the 3 HDMI devices are inherently independent. Each one can
> have different hw rates, bits, and speaker allocation. And if
> implemented as PCM substreams, how can the user space specify "please
> play this music on this monitor"? AFAIK the windows driver also
> exports 3 independent HDMI playback devices.

Yeah, as mentioned, SPDIF status mapping isn't provided properly as
is.  Thus using PCM substreams may give some confusion indeed.


Takashi