Re: [PATCH 7/15] dt-bindings: display: sun4i: Add allwinner,tcon-channel property

Maxime Ripard
2017-03-26T21:11:22Z

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On Fri, Mar 17, 2017 at 11:34:45AM +0800, Chen-Yu Tsai wrote:
> On Thu, Mar 16, 2017 at 1:37 AM, Rob Herring <robh@kernel.org> wrote:
> > On Tue, Mar 07, 2017 at 09:56:26AM +0100, Maxime Ripard wrote:
> >> The Allwinner Timings Controller has two, mutually exclusive, channels.
> >> When the binding has been introduced, it was assumed that there would be
> >> only a single user per channel in the system.
> >>
> >> While this is likely for the channel 0 which only connects to LCD displays,
> >> it turns out that the channel 1 can be connected to multiple controllers in
> >> the SoC (HDMI and TV encoders for example). And while the simultaneous use
> >> of HDMI and TV outputs cannot be achieved, switching from one to the other
> >> at runtime definitely sounds plausible.
> >>
> >> Add an extra property, allwinner,tcon-channel, to specify for a given
> >> endpoint which TCON channel it is connected to, while falling back to the
> >> previous mechanism if that property is missing.
> >
> > I think perhaps TCON channels should have been ports rather than
> > endpoints. The fact that the channels are mutually exclusive can be
> > handled in the driver and doesn't really matter in the binding. How
> > painful would it be to rework things to move TCON channel 1 from port 0,
> > endpoint 1 to port 1?
>
> Having a separate output port for channel 1 was one option we discussed.
> However it wouldn't work well with the kernel's of_graph based crtc
> detection (drm_of_find_possible_crtcs / drm_of_find_possible_crtcs),
> which has the crtcs bound via the output port. As the logic is used
> by multiple drivers, I'm not sure it's easy to rework or test.

Can't we use a different logic than drm_of_find_possible_crtcs to fill
the same role?

> Also, we still have to support old device trees using channel 1 from
> output port 0 endpoint 1. This is the TV encoder on sun5i (A10s/A13/R8).

We could probably work something out if we go that way to deal with
old DTs though.

Maxime

--
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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