[alsa-devel] Timing issues between ALSA and i915 drivers

[Takashi Iwai]

On Wed, 16 Jan 2019 18:48:25 +0100,
Pierre-Louis Bossart wrote:
> 
> 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)

This isn't guaranteed to work, as request_module() might be involved,
I'm afraid.

> 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?

snd_hdac_i915_init() waits for the binding with the i915 audio
component, so a possible solution would be just to delay the audio
component registration at the really last stage, like the change
below.

If this still doesn't work, it'll be more deeply inside, and I have
little clue for now...


thanks,

Takashi

---
diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -1577,8 +1577,6 @@ static void i915_driver_register(struct drm_i915_private *dev_priv)
 	if (IS_GEN5(dev_priv))
 		intel_gpu_ips_init(dev_priv);
 
-	intel_audio_init(dev_priv);
-
 	/*
 	 * Some ports require correctly set-up hpd registers for detection to
 	 * work properly (leading to ghost connected connector status), e.g. VGA
@@ -1597,6 +1595,8 @@ static void i915_driver_register(struct drm_i915_private *dev_priv)
 
 	intel_power_domains_enable(dev_priv);
 	intel_runtime_pm_enable(dev_priv);
+
+	intel_audio_init(dev_priv);
 }
 
 /**