Re: Armada DRM: bridge with componentized devices

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Thu, Jan 03, 2019 at 10:47:27AM +0100, Lubomir Rintel wrote:
> Hello,
> 
> lately I've been trying to make the Himax HX8837 chip that drives the OLPC
> LCD display work with Armada DRM driver. I've been advised to create a
> bridge driver and not an encoder driver since the silicon is separate from
> the LCDC.
> 
> The Armada DRM driver (and, I think, the i.MX one) creates the drm_device
> once the component infrastructure sees the necessary sub-devices appear.
> The sub-devices being the LCDCs and the encoders (not bridges) that it
> expects to be created externally.
> 
> Currently, it seems, the only driver that can actually work with this (that
> is -- creates a drm_encoder for a drm_device when the component is bound)
> is the tda998x. All other similar drivers create a drm_bridge instead and
> not use the component infrastructure at all. (In fact, tilcdc driver
> contains a  hack to handle tda998x specially.)
> 
> I'm wondering how to reconcile the two?
> 
> * The tda998x driver has recently been modified to create a bridge on probe
>   and eventually encoder on component bind. Is this an okay thing to do in
>   a new driver? (this probably means the tilcdc hack can be removed...)
> 
> * If a non-componentized bridge were to be used (along with a dummy 
>   encoder), at what point would it make sense to look for the bridge? 
>   Would it be a good idea to defer the probe of crtc until a bridge can be 
>   looked up and the attach it on component bind?  What if the bridge goes 
>   away (a module is unloaded, etc.) in between?
> 
> I'd be thankful for opintions and advice before I move ahead with this.

This is the long-standing problem with the conflict between bridge
support and component support, and I'm not sure that there is really
any answer to it.

I've gone into the details of the two several times on the list,
particularly about the short-comings of the bridge approach, but it
seems no one cares to fix those short-comings.

You are re-identifying some of the issues that I've already pointed
out - such as what happens to DRM drives when the bridge driver is
unbound (it's really not about modules being unloaded, and the problem
can't be solved by taking a module reference count - all that the
module reference count does is ensure that the module doesn't go
away unexpected, there is no way to ensure that the device isn't
unbound.)

The issue of unbinding is precisely the issue which the component
support was created to solve - but everyone seems to prefer the buggy
bridge approach, and no one seems willing to do anything about the
bugs or even acknowledge that it's a problem.  It's strange - if one
identifies bugs that result in kernel oops in other kernel subsystems,
one is generally taken seriously and the problem is solved.

The issue about the encoders is something that I've tried to discuss,
and I've pointed out that moving it into the DRM driver adds additional
complexity there, and I'd hoped that my patch set I posted would've
generated discussion, but alas not.

What I'm not prepared to do is to introduce _known_ bugs into any
driver that I maintain.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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