Hi,
I could use some feedback on HDMI audio issues exposed during the 4.21
merge window. By accident (misleading documentation) we ended up
enabling the Skylake driver instead of the HDaudio legacy, and broke
audio on a number of Skylake and ApolloLake devices where the HDMI/iDISP
codec was not detected (bit 2 not set in the codec_mask). Linus' Dell
XPS13 9350 was the first to be impacted of course...
After debugging a bit, this issue can be resolved by either
a) compiling both SOUND and DRM as built-ins (y instead of m)
b) moving the calls snd_hdac_i915_init() to the probe function instead
of the worker queue (code at
https://github.com/plbossart/sound/commits/fix/skl-hdmi)
Both solutions point to timing issues.
During internal reviews I was alerted to the fact that the suggested fix
essentially reverts patch ab1b732d53c18 ('ASoC: Intel: Skylake: Move
i915 registration to worker thread') which was introduced to solve DRM
lockup issues.
In other words, we are at a point where we either have DRM lockups or
can't detect the HDMI audio codec, none of which are too good.
I don't have the background for the DRM lockup stuff, nor do I
understand why snd_hdac_i915_init has to be called from a thread
context. Is this really a requirement?
I also don't get what sort of timing issues would come from an
initialization. What happens on the i915 side and is there some sort of
mandatory delay between the completion of the i915_init and issuing a
snd_hdac_chip_readw(bus, STATESTS) to get the codec masks on the HDaudio
links?
Thanks for any pointers or comments.
-Pierre
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