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From:   Jernej =?utf-8?B?xaBrcmFiZWM=?= <jernej.skrabec@siol.net>
To:     Maxime Ripard <maxime.ripard@bootlin.com>
Cc:     Chen-Yu Tsai <wens@csie.org>, Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        devicetree <devicetree@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-clk <linux-clk@vger.kernel.org>,
        linux-sunxi <linux-sunxi@googlegroups.com>
Subject: Re: [PATCH 06/15] drm/sun4i: tcon: Add support for tcon-top
Date:   Thu, 31 May 2018 19:54:08 +0200
Message-ID: <2293030.uW1p7mJGcj@jernej-laptop>
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Dne četrtek, 31. maj 2018 ob 11:21:33 CEST je Maxime Ripard napisal(a):
> On Thu, May 24, 2018 at 03:01:09PM -0700, Chen-Yu Tsai wrote:
> > >> > > + if (tcon->quirks->needs_tcon_top) {
> > >> > > +         struct device_node *np;
> > >> > > +
> > >> > > +         np = of_parse_phandle(dev->of_node, "allwinner,tcon-top",
> > >> > > 0);
> > >> > > +         if (np) {
> > >> > > +                 struct platform_device *pdev;
> > >> > > +
> > >> > > +                 pdev = of_find_device_by_node(np);
> > >> > > +                 if (pdev)
> > >> > > +                         tcon->tcon_top =
> > >> > > platform_get_drvdata(pdev);
> > >> > > +                 of_node_put(np);
> > >> > > +
> > >> > > +                 if (!tcon->tcon_top)
> > >> > > +                         return -EPROBE_DEFER;
> > >> > > +         }
> > >> > > + }
> > >> > > +
> > >> > 
> > >> > I might have missed it, but I've not seen the bindings additions for
> > >> > that property. This shouldn't really be done that way anyway, instead
> > >> > of using a direct phandle, you should be using the of-graph, with the
> > >> > TCON-top sitting where it belongs in the flow of data.
> > >> 
> > >> Just to answer to the first question, it did describe it in "[PATCH
> > >> 07/15] dt- bindings: display: sun4i-drm: Add R40 HDMI pipeline".
> > >> 
> > >> As why I designed it that way - HW representation could be described
> > >> that way> >> 
> > >> (ASCII art makes sense when fixed width font is used to view it):
> > >>                             / LCD0/LVDS0
> > >>                 
> > >>                 / TCON-LCD0
> > >>                 
> > >>                 |           \ MIPI DSI
> > >> 
> > >> mixer0          |
> > >> 
> > >>        \        / TCON-LCD1 - LCD1/LVDS1
> > >>        
> > >>         TCON-TOP
> > >>        
> > >>        /        \ TCON-TV0 - TVE0/RGB
> > >> 
> > >> mixer1          |          \
> > >> 
> > >>                 |           TCON-TOP - HDMI
> > >>                 |          
> > >>                 |          /
> > >>                 
> > >>                 \ TCON-TV1 - TVE1/RGB
> > >> 
> > >> This is a bit simplified, since there is also TVE-TOP, which is
> > >> responsible
> > >> for sharing 4 DACs between both TVE encoders. You can have two TV outs
> > >> (PAL/ NTSC) or TVE0 as TV out and TVE1 as RGB or vice versa. It even
> > >> seems that you can arbitrarly choose which DAC is responsible for
> > >> which signal, so there is a ton of possible end combinations, but I'm
> > >> not 100% sure.
> > >> 
> > >> Even though I wrote TCON-TOP twice, this is same unit in HW. R40 manual
> > >> suggest more possibilities, although some of them seem wrong, like RGB
> > >> feeding from LCD TCON. That is confirmed to be wrong when checking BSP
> > >> code.
> > >> 
> > >> Additionally, TCON-TOP comes in the middle of TVE0 and LCD0, TVE1 and
> > >> LCD1 for pin muxing, although I'm not sure why is that needed at all,
> > >> since according to R40 datasheet, TVE0 and TVE1 pins are dedicated and
> > >> not on PORT D and PORT H, respectively, as TCON-TOP documentation
> > >> suggest. However, HSYNC and PSYNC lines might be shared between TVE
> > >> (when it works in RGB mode) and LCD. But that is just my guess since
> > >> I'm not really familiar with RGB and LCD interfaces.
> > >> 
> > >> I'm really not sure what would be the best representation in OF-graph.
> > >> Can you suggest one?
> > > 
> > > Rob might disagree on this one, but I don't see anything wrong with
> > > having loops in the graph. If the TCON-TOP can be both the input and
> > > output of the TCONs, then so be it, and have it described that way in
> > > the graph.
> > > 
> > > The code is already able to filter out nodes that have already been
> > > added to the list of devices we need to wait for in the component
> > > framework, so that should work as well.
> > > 
> > > And we'd need to describe TVE-TOP as well, even though we don't have a
> > > driver for it yet. That will simplify the backward compatibility later
> > > on.
> > 
> > I'm getting the feeling that TCON-TOP / TVE-TOP is the glue layer that
> > binds everything together, and provides signal routing, kind of like
> > DE-TOP on A64. So the signal mux controls that were originally found
> > in TCON0 and TVE0 were moved out.
> > 
> > The driver needs to know about that, but the graph about doesn't make
> > much sense directly. Without looking at the manual, I understand it to
> > likely be one mux between the mixers and TCONs, and one between the
> > TCON-TVs and HDMI. Would it make more sense to just have the graph
> > connections between the muxed components, and remove TCON-TOP from
> > it, like we had in the past? A phandle could be used to reference
> > the TCON-TOP for mux controls, in addition to the clocks and resets.
> > 
> > For TVE, we would need something to represent each of the output pins,
> > so the device tree can actually describe what kind of signal, be it
> > each component of RGB/YUV or composite video, is wanted on each pin,
> > if any. This is also needed on the A20 for the Cubietruck, so we can
> > describe which pins are tied to the VGA connector, and which one does
> > R, G, or B.
> 
> I guess we'll see how the DT maintainers feel about this, but my
> impression is that the OF graph should model the flow of data between
> the devices. If there's a mux somewhere, then the data is definitely
> going through it, and as such it should be part of the graph.

I concur, but I spent few days thinking how to represent this sanely in graph, 
but I didn't find any good solution. I'll represent here my idea and please 
tell your opinion before I start implementing it.

First, let me be clear that mixer0 and mixer1 don't have same capabilities 
(different number of planes, mixer0 supports writeback, mixer1 does not, 
etc.). Thus, it does matter which mixer is connected to which TCON/encoder. 
mixer0 is meant to be connected to main display and mixer1 to auxiliary. That 
obviously depends on end system.

So, TCON TOP has 3 muxes, which have to be represented in graph. Two of them 
are for mixer/TCON relationship (each of them 1 input and 4 possible outputs) 
and one for TV TCON/HDMI pair selection (2 possible inputs, 1 output).

According to current practice in sun4i-drm driver, graph has to have port 0, 
representing input and port 1, representing output. This would mean that graph 
looks something like that:

tcon_top: tcon-top@1c70000 {
	compatible = "allwinner,sun8i-r40-tcon-top";
	...
	ports {
		#address-cells = <1>;
		#size-cells = <0>;

		tcon_top_in: port@0 {
			#address-cells = <1>;
			#size-cells = <0>;
			reg = <0>;

			tcon_top_in_mixer0: endpoint@0 {
				reg = <0>;
				remote-endpoint = <&mixer0_out_tcon_top>;
			};

			tcon_top_in_mixer1: endpoint@1 {
				reg = <1>;
				remote-endpoint = <&mixer1_out_tcon_top>;
			};

			tcon_top_in_tcon_tv: endpoint@2 {
				reg = <2>;
				// here is HDMI input connection, part of board DTS
				remote-endpoint = <board specific phandle to TV TCON output>;
			};
		};

		tcon_top_out: port@1 {
			#address-cells = <1>;
			#size-cells = <0>;
			reg = <1>;

			tcon_top_out_tcon0: endpoint@0 {
				reg = <0>;
				// here is mixer0 output connection, part of board DTS
				remote-endpoint = <board specific phandle to TCON>;
			};

			tcon_top_out_tcon1: endpoint@1 {
				reg = <1>;
				// here is mixer1 output connection, part of board DTS
				remote-endpoint = <board specific phandle to TCON>;
			};

			tcon_top_out_hdmi: endpoint@2 {
				reg = <2>;
				remote-endpoint = <&hdmi_in_tcon_top>;
			};
		};
	};
};

tcon_tv0: lcd-controller@1c73000 {
	compatible = "allwinner,sun8i-r40-tcon-tv-0";
	...
	ports {
		#address-cells = <1>;
		#size-cells = <0>;

		tcon_tv0_in: port@0 {
			reg = <0>;

			tcon_tv0_in_tcon_top: endpoint {
				// endpoint depends on board, part of board DTS
				remote-endpoint = <phandle to one of tcon_top_out_tcon>;
			};
		};

		tcon_tv0_out: port@1 {
			#address-cells = <1>;
			#size-cells = <0>;
			reg = <1>;

			// endpoints to TV TOP and TCON TOP HDMI input
			...
		};
	};
};

tcon_tv1: lcd-controller@1c74000 {
	compatible = "allwinner,sun8i-r40-tcon-tv-1";
	...
	ports {
		#address-cells = <1>;
		#size-cells = <0>;

		tcon_tv1_in: port@0 {
			reg = <0>;

			tcon_tv1_in_tcon_top: endpoint {
				// endpoint depends on board, part of board DTS
				remote-endpoint = <phandle to one of tcon_top_out_tcon>;
			};
		};

		tcon_tv1_out: port@1 {
			#address-cells = <1>;
			#size-cells = <0>;
			reg = <1>;

			// endpoints to TV TOP and TCON TOP HDMI input
			...
		};
	};
};

tcon_lcd0 and tcon_lcd1 would have similar connections, except that for 
outputs would be LCD or LVDS panels or MIPI DSI encoder.

Please note that each TCON (there are 4 of them) would need to have unique 
compatible and have HW index stored in quirks data. It would be used by TCON 
TOP driver for configuring muxes.

What do you think about above suggestion?

Best regards,
Jernej