[alsa-devel] [PATCH 00/11] drm/i915: LPE audio runtime PM and multipipe

[Ville Syrjälä]

On Wed, Apr 26, 2017 at 03:47:44PM +0200, Takashi Iwai wrote:
> On Wed, 26 Apr 2017 15:38:53 +0200,
> Ville Syrjälä wrote:
> > 
> > On Wed, Apr 26, 2017 at 09:29:18AM +0200, Takashi Iwai wrote:
> > > On Tue, 25 Apr 2017 22:27:19 +0200,
> > > ville.syrjala at linux.intel.com wrote:
> > > > 
> > > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > > 
> > > > I was wondering why my VLV no longer runtime suspended, and after some
> > > > thinking I decided it had to be the LPE audio preventing it. Turns out
> > > > I was right, so here's my attempt at fixing it.
> > > > 
> > > > And while looking at the code I couldn't help but notice that it
> > > > couldn't actually handle multiple pipes playing back audio at the
> > > > same time. And even having multiple displays active even if only
> > > > one was playing audio was probably a recipe for failure. So I
> > > > tried to fix that by registering a separate PCM device for each
> > > > pipe.
> > > > 
> > > > Note that the patch subjects may not reflect the subsystem
> > > > very well since most of these straddle the border between drm
> > > > and alsa. I think I just slapped on drm/i915 to most where
> > > > there was no clear winner.
> > > 
> > > A nice patchset, thanks for working on it!
> > > 
> > > One slight concern (other than the jack issue Pierre reported) is the
> > > incompatible behavior from the current version.  With the pipe-based
> > > multiple streams, user would need to choose another one even if the
> > > device has a single HDMI output, which is pretty common on BYT/CHV
> > > tablets.
> > > 
> > > Maybe it's no big problem as the users are still limited at the
> > > moment.  Or, we may need to handle a bit differently, e.g. assigning
> > > the PCM stream dynamically per hotplug.
> > 
> > Yeah, I tied the PCM device to the pipe to match the hardware. But we
> > could certainly register the PCM device per port, and then do a 
> > pipe<->port mapping somewhere to make it all work out. One slight
> > complication is that not all ports are necessarily present so we
> > might have eg. just port B and port D, but no port C. Not sure if
> > it would be a bad thing to register a PCM device even for the
> > missing ports anyway?
> > 
> > I don't recall which way HDA works. Device per port I guess? Well,
> > for DP SST/HDMI. But for DP MST I presume it's device per stream
> > (ie. pipe) even with HDA.
> 
> HD-audio creates the PCM device per HD-audio widget representation,
> and they correspond to ports, as it seems.

With the slight exception of old stuff like ctg/elk where there is just
one audio device IIRC, and that one gets fed into whichever port enables
audio, and if both enable it it goes to /dev/null.

> For MST, the situation is
> more complex, and we assign the PCM stream dynamically.  It's for
> keeping the behavior (more or less) consistent with non-MST.
> 
> > > In anyway, with the support of multi streams, alsa-lib config needs to
> > > be updated.
> > 
> > Hmm. I suppose I'll have to take a gander at what's there.
> 
> The HDMI PCM definition itself is easy, just just adding more (hdmi.1,
> hdmi.2, etc).  The difficult part would be the front and the default
> PCM definition, and I have no good idea about it, so far.
> 
> 
> Takashi

-- 
Ville Syrjälä
Intel OTC