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From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Daniel Vetter <daniel@ffwll.ch>
Cc:     Peter Rosin <peda@axentia.se>, linux-kernel@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        Boris Brezillon <boris.brezillon@free-electrons.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        devicetree@vger.kernel.org, David Airlie <airlied@linux.ie>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        dri-devel@lists.freedesktop.org, Rob Herring <robh+dt@kernel.org>,
        Jacopo Mondi <jacopo@jmondi.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 0/5] allow override of bus format in bridges
Date:   Wed, 04 Apr 2018 01:28:29 +0300
Message-ID: <2233231.my6BbD3pcT@avalon>
Organization: Ideas on Board Oy
In-Reply-To: <20180328070826.GY14155@phenom.ffwll.local>
References: <20180326212447.7380-1-peda@axentia.se> <20180328070826.GY14155@phenom.ffwll.local>
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Hi Daniel,

On Wednesday, 28 March 2018 10:08:26 EEST Daniel Vetter wrote:
> On Mon, Mar 26, 2018 at 11:24:42PM +0200, Peter Rosin wrote:
> > Hi!
> > 
> > [I got to v2 sooner than expected]
> > 
> > I have an Atmel sama5d31 hooked up to an lvds encoder and then
> > on to an lvds panel. Which seems like something that has been
> > done one or two times before...
> > 
> > The problem is that the bus_format of the SoC and the panel do
> > not agree. The SoC driver (atmel-hlcdc) can handle the
> > rgb444, rgb565, rgb666 and rgb888 bus formats. The hardware is
> > wired for the rgb565 case. The lvds encoder supports rgb888 on
> > its input side with the LSB wires for each color simply pulled
> > down internally in the encoder in my case which means that the
> > rgb565 bus_format is the format that works best. And the panel
> > is expecting lvds (vesa-24), which is what the encoder outputs.
> > 
> > The reason I "blame" the bus_format of the drm_connector is that
> > with the below DT snippet, things do not work *exactly* due to
> > that. At least, it starts to work if I hack the panel-lvds driver
> > to report the rgb565 bus_format instead of vesa-24.
> > 
> > 	panel: panel {
> > 		compatible = "panel-lvds";
> > 		
> > 		width-mm = <304>;
> > 		height-mm = <228;
> > 		
> > 		data-mapping = "vesa-24";
> > 		
> > 		panel-timing {
> > 			// 1024x768 @ 60Hz (typical)
> > 			clock-frequency = <52140000 65000000 71100000>;
> > 			hactive = <1024>;
> > 			vactive = <768>;
> > 			hfront-porch = <48 88 88>;
> > 			hback-porch = <96 168 168>;
> > 			hsync-len = <32 64 64>;
> > 			vfront-porch = <8 13 14>;
> > 			vback-porch = <8 13 14>;
> > 			vsync-len = <8 12 14>;
> > 		};
> > 		
> > 		port {
> > 			panel_input: endpoint {
> > 				remote-endpoint = <&lvds_encoder_output>;
> > 			};
> > 		};
> > 	};
> > 	
> > 	lvds-encoder {
> > 		compatible = "ti,ds90c185", "lvds-encoder";
> > 		
> > 		ports {
> > 			#address-cells = <1>;
> > 			#size-cells = <0>;
> > 			
> > 			port@0 {
> > 				reg = <0>;
> > 				
> > 				lvds_encoder_input: endpoint {
> > 					remote-endpoint = <&hlcdc_output>;
> > 				};
> > 			};
> > 			
> > 			port@1 {
> > 				reg = <1>;
> > 				
> > 				lvds_encoder_output: endpoint {
> > 					remote-endpoint = <&panel_input>;
> > 				};
> > 			};
> > 		};
> > 	};
> > 
> > But, instead of perverting the panel-lvds driver with support
> > for a totally fake non-lvds bus_format, I intruduce an API that allows
> > display controller drivers to query the required bus_format of any
> > intermediate bridges, and match up with that instead of the formats
> > given by the drm_connector. I trigger this with this addition to the
> > 
> > lvds-encoder DT node:
> > 		interface-pix-fmt = "rgb565";
> > 
> > Naming is hard though, so I'm not sure if that's good?
> > 
> > I threw in the first patch, since that is the actual lvds encoder
> > I have in this case.
> > 
> > Suggestions welcome.
> 
> Took a quick look, feels rather un-atomic. And there's beend discussing
> for other bridge related state that we might want to track (like the full
> adjusted_mode that might need to be adjusted at each stage in the chain).
> So here's my suggestions:
> 
> - Add an optional per-bridge internal state struct using the support in
> 
> https://dri.freedesktop.org/docs/drm/gpu/drm-kms.html#handling-driver-privat
> e-state
> 
>   Yes it says "driver private", but since bridge is just helper stuff
>   that's all included. "driver private" == "not exposed as uapi" here.
>   Include all the usual convenience wrappers to get at the state for a
>   bridge.
> 
> - Then stuff your bus_format into that new drm_bridge_state struct.
> 
> - Add a new bridge callback atomic_check, which gets that bridge state as
>   parameter (similar to all the other atomic_check functions).
> 
> This way we can even handle the bus_format dynamically, through the atomic
> framework your bridge's atomic_check callback can look at the entire
> atomic state (both up and down the chain if needed), it all neatly fits
> into atomic overall and it's much easier to extend.

While I think we'll eventually need bridge states, I don't think that's need 
yet. The bus formats reported by this patch series are static. We're not 
talking about the currently configured format for a bridge, but about the list 
of supported formats. This is similar to the bus_formats field present in the 
drm_display_info structure. There is thus in my opinion no need to interface 
this with atomic until we need to track the current format (and I think that 
will indeed happen at some point, but I don't think Peter needs this feature 
for now). That's why I've told Peter that I would like a bridge API to report 
the information and haven't requested a state-based implementation.

> Please also cc Laurent Pinchart on this.
> 
> > Changes since v1 https://lkml.org/lkml/2018/3/17/221
> > - Add a proper bridge API to query the bus_format instead of abusing
> >   the ->get_modes part of the code. This is cleaner but requires
> >   changes to all display controller drivers wishing to participate.
> > - Add patch to adjust the atmel-hlcdc driver according to the above.
> > - Hook the new info into the bridge local to the lvds-encoder instead
> >   of messing about with new interfaces for the panel-bridge driver.
> > - Add patch with a DT parsing function of bus_formats in a central place.
> > - Rephrase the addition of ti,ds90c185 to the lvds-transmitter binding.
> > 
> > Peter Rosin (5):
> >   dt-bindings: display: bridge: lvds-transmitter: add ti,ds90c185
> >   drm: bridge: add API to query the expected input formats of bridges
> >   drm: of: add display bus-format parser
> >   drm: bridge: lvds-encoder: allow specifying the input bus format
> >   drm/atmel-hlcdc: take bridges into account when selecting output
> >     format
> >  
> >  .../bindings/display/bridge/lvds-transmitter.txt   | 14 ++++-
> >  .../devicetree/bindings/display/bus-format.txt     | 35 +++++++++++++
> >  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c     | 49 +++++++++++++++--
> >  drivers/gpu/drm/bridge/lvds-encoder.c              | 25 +++++++++
> >  drivers/gpu/drm/drm_bridge.c                       | 32 ++++++++++++
> >  drivers/gpu/drm/drm_of.c                           | 59 +++++++++++++++++
> >  include/drm/drm_bridge.h                           | 18 +++++++
> >  include/drm/drm_of.h                               |  9 ++++
> >  8 files changed, 237 insertions(+), 4 deletions(-)
> >  create mode 100644
> >  Documentation/devicetree/bindings/display/bus-format.txt

-- 
Regards,

Laurent Pinchart