[alsa-devel] HDMI codec, way forward?

[Russell King - ARM Linux]

On Wed, Oct 21, 2015 at 03:41:44PM +0200, Takashi Iwai wrote:
> On Wed, 21 Oct 2015 11:27:44 +0200,
> Russell King - ARM Linux wrote:
> > 
> > On Tue, Oct 20, 2015 at 07:31:48PM +0530, Vinod Koul wrote:
> > > On Tue, Oct 20, 2015 at 09:08:00AM +0100, Russell King - ARM Linux wrote:
> > > > > > Currently i915/audio component works as you described.  The audio is
> > > > > > optional and HDMI graphics works without audio, while HDMI HD-audio
> > > > > > mandates i915 graphics.
> > > > > 
> > > > > Right, but we also add additional interface on top of this to allow
> > > > > things like ensuring display is on when audio wants to run and now
> > > > > notification for events.
> > > > > 
> > > > > I don't see a reason why this can be used as single interface to bind as
> > > > > well as talk to display from various components, unless I missed obvious
> > > > > which prevent us from doing this in non i915 cases...
> > > > 
> > > > Maybe I can comment more specifically if I saw the code.  Right now all
> > > > I'm aware of is this idea without any code, and I don't like it.
> > > 
> > > Ok, i will be post my patches tomorrow. FWIW uses interface in
> > > sound/hda/hdac_i915.c for display power up/down
> > 
> > This:
> > 
> >         component_match_add(dev, &match, hdac_component_master_match, bus);
> >         ret = component_master_add_with_match(dev, &hdac_component_master_ops,
> >                                               match);
> >         if (ret < 0)
> >                 goto out_err;
> > 
> >         /*
> >          * Atm, we don't support deferring the component binding, so make sure
> >          * i915 is loaded and that the binding successfully completes.
> >          */
> >         request_module("i915");
> > 
> >         if (!acomp->ops) {
> >                 ret = -ENODEV;
> >                 goto out_master_del;
> >         }
> > 
> > is really not nice.
> 
> Yeah, admittedly it's a really ugly workaround.  It's coded in that
> way just to make our lives easier: it works well in most cases for
> i915 / hd-audio.
> 
> Actually, it's easy to modify the hda code in the right way, i.e. to
> defer via component bind ops.  However, one drawback is that it's not
> trivial to handle the fallback case; namely, HD-audio might not need
> i915 binding, depending on the chip model and the configuration.
> 
> Braswell and Skylake have a (virtually) single audio controller as
> default, and this manages both the analog and HDMI/DP codecs.  The
> i915 interaction is required only for the latter codecs, and thus for
> the former, it's optional.  If we defer binding, the instantiation
> won't happen unless it's bound with i915.  So, if i915 isn't
> initialized (e.g. by booting with nomodeset option), the whole audio
> will be gone, too.  OTOH, Haswell and Braswell have a dedicated HDA
> controller for HDMI/DP, and they must be disabled (or fail) unless
> bound with graphics.
> 
> It's a corner case, so we might better ignore this.  But it'd be
> certainly a "regression" -- at least, I used to test this in that way
> sometimes in the past.  So it remains in the current way as is.
> 
> It's one of the reasons I mentioned it being a stop gap.  But, I think
> the concept, passing ops via component, is actually working, and
> should be applicable to other drm/audio drivers.  That's the point.

It's only the point if you can code it up properly, which from what I
read in that file, it isn't.

Build the i915 DRM drivers as a module, load up the system, and then
try removing the i915 DRM module and see what happens to the audio part.
For starters, you have no protection what so ever against acomp->ops or
acomp->dev becoming NULL - it's hellishly racy.

Secondly, you reject the initialisation if acomp->ops isn't set, but you
allow acomp->ops to later become unset by the i915 DRM module being
removed.  If you can cope with acomp->ops being unset at a random point
during the audio driver's use, why can't you cope with it being set at
some random point later?

I don't think this code has been thought through at all.

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