On Fri, 23 Aug 2019 16:29:40 +0200, Kai Vehmanen wrote:
Hi,
On Thu, 15 Aug 2019, Takashi Iwai wrote:
Kai Vehmanen wrote:
into modifying the SOF Intel backend to use snd-hda-codec-hdmi/patch_hdmi.c for HDMI/DP audio support, i.e. to be able to share this code between snd-hda-intel and SOF Intel (and not using hdac-hdmi).
[..]
This will change how HDMI is exposed to user-space with SOF Intel drivers, so we need to be extra careful how this is introduced. But
Agreed. I guess the biggest difference is the handling of the DP-MST. The legacy HD-audio HDMI driver takes a different approach for DP-MST, namely, it chooses dynamically the pin that is connected with a monitor. It's for keeping the compatibility (more or less) with old behavior; the program just needs to open the device that corresponds to the notification via jack ctl without fiddling with other extra routing.
indeed. I've now got to a point where I have the key functionality in place. And after some reworks, the changes to patch_hdmi.c are pretty minimal, which is very nice (I started with a much more evasive patch).
But, but. The DP-MST handling is indeed iffy. I tried a few approaches, but it is hard to reconcile concept of "backup PCMs" of patch_hdmi.c with concepts of ASoC and ALSA topology, where the PCM and DAIs are supposed to be defined in the topology file. This gets worse with SOF (and any similar usage) which allow you to have arbitrary DSP processing between a PCM and the HDMI/DP DAIs. So this seems like a dead-end.
What I ended up doing was to make a new mode to patch_hdmi.c that limits PCM count to actual converter count (and this is aligned with topology), and still supporting DP-MST by always mapping monitors to a free PCM. I've tested some complex DP-MST scenarios and this seems to work pretty well. The jack detection will still be able to tell which of the PCMs have a monitor detected.
I wonder if this would be an acceptable approach, given the reuse benefits we get. Downsides:
- assignment of monitors to PCMs will depend on ELD update ordering
This is true in anyway for the legacy HD-audio, too.
- in SOF we need to align PCM numbering scheme in all topologies we convert over to patch_hdmi.c
I don't think this can be a problem.
Practically seen, the number of PCMs could be just 3 on Skylake and co, even with DP MST, because i915 can drive only up to 3 monitors. The driver *should* receive the unplug event before the plug event to a new monitor, so hda_detach_hda_pcm() gets called before hda_attach_hda_pcm().
The current patch_hdmi.c implementation is based on the theoretical possibility, and limitation to the reduced PCM streams would work, I suppose.
thanks,
Takashi
I did not yet figure out how to toggle the new DP-MST mode in patch_hdmi.c in a nice way, so didn't sent patches to list yet. I'll do that early next week. If you want a sneak preview, I uploaded the series at:
https://github.com/thesofproject/linux/pull/1155
Br, Kai