[alsa-devel] [PATCH] conf: HdmiLpeAudio: remove the "front" pcm definition

[Tanu Kaskinen]

On Thu, 2017-10-05 at 08:38 +0200, Takashi Iwai wrote:
> On Wed, 04 Oct 2017 21:44:00 +0200,
> Tanu Kaskinen wrote:
> > 
> > PulseAudio assumes that the "front" pcm device always refers to an
> > analog device, not HDMI. While that assumption is not really valid, the
> > reality is that without that assumption PulseAudio can't know whether
> > "front" and "hdmi" refer to a different or the same device.
> > 
> > The HDMI LPE driver doesn't allow audio streaming while the HDMI cable
> > is unplugged, so PulseAudio has to know when it's plugged in and when
> > it's not. If both "front" and "hdmi" devices exist, PulseAudio will
> > notice that HDMI is unplugged, but it doesn't know that "front" refers
> > to the same device, and PulseAudio will try to use the "front" device
> > with bad consequences. The kernel driver's refusal to stream any audio
> > makes PulseAudio enter an infinite loop and then the kernel kills
> > PulseAudio, because it consumes too much CPU time in a realtime thread.
> > 
> > While the looping in PulseAudio could probably be fixed, that wouldn't
> > change the fact that PulseAudio thinks that there is an analog device. I
> > believe it's best to avoid having the same device as both "front" and
> > "hdmi" in alsa-lib.
> > 
> > I removed also the surround configuration includes. I don't think they
> > had any effect anyway, so I wonder why they were there in the first
> > place.
> 
> These definitions are there for LPCM surround channels.

Are they supposed to do something useful? When PulseAudio probes the
surround40:0 device, opening the device fails:

alsa-util.c: Trying surround40:0 with SND_PCM_NO_AUTO_FORMAT ...
(alsa-lib)confmisc.c: Unable to find definition 'cards.HdmiLpeAudio.pcm.surround40.0:CARD=0'
(alsa-lib)conf.c: function snd_func_refer returned error: No such file or directory
(alsa-lib)conf.c: Evaluate error: No such file or directory
(alsa-lib)pcm.c: Unknown PCM surround40:0

> The biggest problem behind the scene is that we mixed up both logical
> and physical configurations.  The "front" or "surround51" belong to
> the former, while "hdmi" belongs to the latter.
> 
> In the past, there was only SPDIF that has only two stereo channels,
> so the logical configuration was fixed with that.  We copied that
> config for HDMI/DP, and now we hit the inconsistency more clearly...
> 
> I suppose the PA issue gets fixed just by this removal?  If yes, I'm
> willing to apply it, including in the next release.

A user reported that removing the "front" definition fixes the PA
issue, together with jack detection patches for PA that haven't been
merged yet.

-- 
Tanu

https://www.patreon.com/tanuk