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

On Tue, Mar 28, 2017 at 06:05:08PM +0800, Icenowy Zheng wrote:

2017?3?27? ??5:11? Maxime Ripard [off-list ref]???

quoted

On Fri, Mar 17, 2017 at 11:34:45AM +0800, Chen-Yu Tsai wrote: 

quoted

On Thu, Mar 16, 2017 at 1:37 AM, Rob Herring [off-list ref] wrote: 

quoted

On Tue, Mar 07, 2017 at 09:56:26AM +0100, Maxime Ripard wrote: 

quoted

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? 

quoted

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). 

And from A83T on we will face channel-1 only TCONs., which do not
have channel 0 at all in hardware.

Yes, but those patches have not been merged yet, so we don't have to
care about the backward compatibility.

Maxime

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