[PATCH 1/2] drm/bridge: Add virtual display DT bindings

[Liviu Dudau]

Hi Andrzej,

Chirping in, as it was originally my work that Linus is adopting.

On Mon, Aug 27, 2018 at 01:53:00PM +0200, Andrzej Hajda wrote:
> On 24.08.2018 14:23, Linus Walleij wrote:
> > This adds bindings for a virtual display to be used with displays
> > inside entirely virtual environments which do not emulate things
> > like monitors but just need timing information to be supplied to
> > its display controller.
> >
> > This is inspired by earlier work by Liviu Dudau.
> 
> If this is pure virtual then it should be connected to other pure
> virtual components.

Any reason why you think this is the only way this should be connected?
My original implementation (and I haven't seen anything in this patch
that would invalidate it) assumed that the virtual bridge can be used by
any driver that uses the component framework. Hiding implementation
details is one of the benefits of using the component framework, so I
don't see what you gain by restricting connection to only other pure
virtual components.

> What will be this virtual bridge attached to? I expect some virtual
> encoder, virtual crtc? If yes then why don't you create whole virtual
> drm pipeline in one patchset?

This comment is probably more relevant to patch 2/2, but I agree that
it probably needs a virtual encoder. As for the CRTC, it should work
with any real DRM driver, AFAICT.

> Could you describe more what do you want to do.

He wants to enable the PL110 DRM driver on systems that lack a way of
detecting the actual encoder and connector being used (either because
there is none and they are emulated in software that doesn't mimic the
HW behaviour, or because the way you access the HW is done in a way that
doesn't model easily in the current DRM framework: by firmware, for
example).

Best regards,
Liviu

> And one more thing, you are defining virtual panel but you are using
> drm_bridge framework, why not drm_panel?
> 
> Regards
> Andrzej
> 
> >
> > Cc: Liviu Dudau <Liviu.Dudau at arm.com>
> > Cc: Ryan Harkin <ryan.harkin at linaro.org>
> > Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
> > ---
> >  .../display/bridge/virtual-display-bridge.txt | 62 +++++++++++++++++++
> >  1 file changed, 62 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/display/bridge/virtual-display-bridge.txt
> >
> > diff --git a/Documentation/devicetree/bindings/display/bridge/virtual-display-bridge.txt b/Documentation/devicetree/bindings/display/bridge/virtual-display-bridge.txt
> > new file mode 100644
> > index 000000000000..ea4f5a91ab94
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/display/bridge/virtual-display-bridge.txt
> > @@ -0,0 +1,62 @@
> > +Virtual Display Bridge
> > +
> > +This represents a display that is contained within an emulated
> > +environment.
> > +
> > +This means that the display engine mainly expects some timing
> > +parameters to be written into it, and after that the emulator will
> > +respond by creating a virtual display with the requested
> > +resolution characteristics.
> > +
> > +As the operating system cannot "detect" such a display, rather the
> > +emulator will respond to what the controller outputs, a
> > +chicken-and-egg problem needs to be solved: the resolution and
> > +timing characteristics need to be defined and set up somewhere.
> > +
> > +The virtual display bridge solves this by defining a bridge with
> > +all timing characteristics encoded into the device tree node.
> > +
> > +Required properies:
> > +- compatible: shall be "virtual-display-bridge"
> > +
> > +Required subnodes:
> > +- display-timings: contains in turn a display timing node
> > +  see display-timing.txt
> > +- ports: contains the display ports, see media/video-interfaces.txt
> > +
> > +Example:
> > +
> > +bridge {
> > +	compatible = "virtual-display-bridge";
> > +	#address-cells = <1>;
> > +	#size-cells = <0>;
> > +
> > +	display-timings {
> > +		/* Some standard VGA timing */
> > +		timing0 {
> > +			clock-frequency = <23750>;
> > +			hactive = <640>;
> > +			vactive = <480>;
> > +			hfront-porch = <48>;
> > +			hback-porch = <16>;
> > +			hsync-len = <96>;
> > +			vfront-porch = <33>;
> > +			vback-porch = <9>;
> > +			vsync-len = <3>;
> > +			vrefresh = <60>;
> > +		};
> > +	};
> > +
> > +	ports {
> > +		#address-cells = <1>;
> > +		#size-cells = <0>;
> > +
> > +		port at 0 {
> > +			reg = <0>;
> > +
> > +			display_bridge_in: endpoint {
> > +				remote-endpoint = <&foo>;
> > +			};
> > +		};
> > +	};
> > +};
> 
> 

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
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