[alsa-devel] [RFC] Fix for conflict of HDMI and SPDIF IEC958 controls

Takashi Iwai tiwai at suse.de
Mon Oct 15 12:31:07 CEST 2012

At Mon, 15 Oct 2012 12:21:18 +0200,
Jaroslav Kysela wrote:
> Date 12.10.2012 17:18, Takashi Iwai wrote:
> > Hi,
> > 
> > there is a long-standing problem in HD-audio regarding IEC958
> > controls.  When both an SPDIF and an HDMI are created on the same
> > card (e.g. one from analog codec and one from graphics chip), the
> > driver assigns the IEC958 controls just with new indices, 0, 1, 2...
> > 
> > The problem is that there is no way to connect between this index and
> > the actual PCM device.  Currently, alsa-lib HDA-Intel.conf has a fixed
> > configuration:
> >   spdif  -> IEC958 xxx index=0, PCM dev=1
> >   hdmi,0 -> IEC958 xxx index=0, PCM dev=3
> >   hdmi,1 -> IEC958 xxx index=1, PCM dev=7
> >   hdmi,2 -> IEC958 xxx index=2, PCM dev=8
> >   hdmi,3 -> IEC958 xxx index=3, PCM dev=9
> > 
> > So obviously spdif and the first hdmi conflict.
> > 
> > Basically this can be fixed by reassigning each IEC958 control with
> > the same device number corresponding to the PCM device.  That is, for
> > SPDIF, assign a control element with device=1, for HDMI,
> > device=3,7...
> > However, this obviously breaks the old configuration unless user
> > upgrades the alsa-lib configuration.  Thus this is no-go.
> > 
> > Now here is a compromise: the IEC958 control for SPDIF is reassigned
> > to device=1 *only* when SPDIF and HDMI coexist.  Since the
> > configuration is anyway broken as is now in such a case, it's no big
> > deal to fix one side in an incompatible way.  (The reason why SPDIF
> > is re-assigned is that I guess majority of user require more HDMI than
> > SPDIF in such a configuration.)
> > 
> > In addition, we need a fix in alsa-lib.  Also not for breaking the
> > compatibility with older kernel, we need some fallback check.  I did a
> > quick hack to alsa-lib conf code and added "skip_rest" option to the
> > hook element, so that only the first matching element is taken.
> > 
> > Long story short, I cooked up three patches.  One patch is for kernel,
> > to add the workaround above, and the two are for alsa-lib, one clean
> > up and one to introduce the skip_rest option (and application to
> > HDA-Intel.conf).
> > 
> > My plan is to merge this to 3.8 tree, so it's no urgent issue.
> > But it's of course always good to fix something.
> What about to introduce more cleaner solution - export this information
> using TLV to user space, so alsa-lib can enumerate (distinguish) the
> spdif and hdmi devices properly.

Yes, this is the next step I've planned.  The additional information
doesn't conflict with this fix.

The point of this fix is to keep / fix the things working as much as
possible even without changing user-space.  If a user-space upgrade is
mandatory, it can be regarded as an obvious regression.

In short, my patch is:
- A quick fix
- Doesn't break the cases without SPDIF/HDMI conflict
- Fixes HDMI for the conflicting case even without changing user-space
- Would break the SPDIF output (only when SPDIF/HDMI conflicts)
  without config update


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