Re: [PATCH 2/7] drm/i915: Add get_eld audio component

From: Daniel Vetter <daniel@ffwll.ch>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: alsa-devel@alsa-project.org,
	Mengdong Lin <mengdong.lin@linux.intel.com>,
	Takashi Iwai <tiwai@suse.de>,
	intel-gfx@lists.freedesktop.org,
	Vinod Koul <vinod.koul@intel.com>,
	Daniel Vetter <daniel.vetter@intel.com>,
	David Henningsson <david.henningsson@canonical.com>
Subject: Re: [PATCH 2/7] drm/i915: Add get_eld audio component
Date: Mon, 30 Nov 2015 17:34:45 +0100	[thread overview]
Message-ID: <20151130163445.GA17050@phenom.ffwll.local> (raw)
In-Reply-To: <20151130160933.GT4437@intel.com>

On Mon, Nov 30, 2015 at 06:09:33PM +0200, Ville Syrjälä wrote:
> On Mon, Nov 30, 2015 at 05:24:41PM +0200, Ville Syrjälä wrote:
> > On Mon, Nov 30, 2015 at 02:37:46PM +0100, Takashi Iwai wrote:
> > > Implement a new i915_audio_component_ops, get_eld().  It's called by
> > > the audio driver to fetch the current ELD of the given HDMI/DP port.
> > > It returns the size of ELD bytes if it's valid, or zero if the audio
> > > is disabled or unplugged, or a negative error code.
> > 
> > Why do we need this? Isn't it something the eld notify hook should
> > pass from i915 to the audio driver?
> > 
> > At least with the locking you have for this, the audio driver can not
> > call this from the eld notify hook since it would deadlock.
> 
> Hmm. Actually the locking isn't perhaps quite like that atm. But I guess
> the mode_config.mutex will make it so.

If we need this we could create a substruct in dev_priv with copies of
everything, which would only be protected by av_mutex. That's the usual
approach we use when faced with this kind of locking inversion:
Copy/update relevant data in the modeset ->enable/disable hooks, then just
use these local locks to access those copies. Usually that's enough to
untangle things, with no need to resort to workers.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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