Re: [PATCH 04/14] media: add V4L2 DT binding documentation

[Sascha Hauer <s.hauer@pengutronix.de>]

On Fri, Oct 05, 2012 at 05:41:00PM +0200, Guennadi Liakhovetski wrote:
> Hi Sascha
> 
> > > +
> > > +	ceu0: ceu@0xfe910000 {
> > > +		compatible = "renesas,sh-mobile-ceu";
> > > +		reg = <0xfe910000 0xa0>;
> > > +		interrupts = <0x880>;
> > > +
> > > +		mclk: master_clock {
> > > +			compatible = "renesas,ceu-clock";
> > > +			#clock-cells = <1>;
> > > +			clock-frequency = <50000000>;	/* max clock frequency */
> > > +			clock-output-names = "mclk";
> > > +		};
> > > +
> > > +		port {
> > > +			#address-cells = <1>;
> > > +			#size-cells = <0>;
> > > +
> > > +			ceu0_1: link@1 {
> > > +				reg = <1>;		/* local link # */
> > > +				remote = <&ov772x_1_1>;	/* remote phandle */
> > > +				bus-width = <8>;	/* used data lines */
> > > +				data-shift = <0>;	/* lines 7:0 are used */
> > > +
> > > +				/* If [hv]sync-active are missing, embedded bt.605 sync is used */
> > > +				hsync-active = <1>;	/* active high */
> > > +				vsync-active = <1>;	/* active high */
> > > +				data-active = <1>;	/* active high */
> > > +				pclk-sample = <1>;	/* rising */
> > > +			};
> > > +
> > > +			ceu0_0: link@0 {
> > > +				reg = <0>;
> > > +				remote = <&csi2_2>;
> > > +				immutable;
> > > +			};
> > > +		};
> > > +	};
> > > +
> > > +	i2c0: i2c@0xfff20000 {
> > > +		...
> > > +		ov772x_1: camera@0x21 {
> > > +			compatible = "omnivision,ov772x";
> > > +			reg = <0x21>;
> > > +			vddio-supply = <&regulator1>;
> > > +			vddcore-supply = <&regulator2>;
> > > +
> > > +			clock-frequency = <20000000>;
> > > +			clocks = <&mclk 0>;
> > > +			clock-names = "xclk";
> > > +
> > > +			port {
> > > +				/* With 1 link per port no need in addresses */
> > > +				ov772x_1_1: link {
> > > +					bus-width = <8>;
> > > +					remote = <&ceu0_1>;
> > > +					hsync-active = <1>;
> > > +					vsync-active = <0>;	/* who came up with an inverter here?... */
> > > +					data-active = <1>;
> > > +					pclk-sample = <1>;
> > > +				};
> > 
> > I currently do not understand why these properties are both in the sensor
> > and in the link. What happens if they conflict? Are inverters assumed
> > like suggested above? I think the bus can only have a single bus-width,
> > why allow multiple bus widths here?
> 
> Yes, these nodes represent port configuration of each party on a certain 
> link. And they can differ in certain properties, like - as you correctly 
> notice - in the case, when there's an inverter on a line. As for other 
> properties, some of them must be identical, like bus-width, still, they 
> have to be provided on both ends, because generally drivers have to be 
> able to perform all the required configuration based only on the 
> information from their own nodes, without looking at "remote" partner node 
> properties.

So the port associated to the ov772x_1 only describes how to configure
the ov772x and it's up to me to make sure that this configuration
matches the partner device. If I don't then it won't work but soc-camera
will happily continue.
Ok, that's good, I thought there would be some kind of matching
mechanism take place here. It may be worth to make this more explicit in
the docs.

> 
> > > +		reg = <0xffc90000 0x1000>;
> > > +		interrupts = <0x17a0>;
> > > +		#address-cells = <1>;
> > > +		#size-cells = <0>;
> > > +
> > > +		port@1 {
> > > +			compatible = "renesas,csi2c";	/* one of CSI2I and CSI2C */
> > > +			reg = <1>;			/* CSI-2 PHY #1 of 2: PHY_S, PHY_M has port address 0, is unused */
> > > +
> > > +			csi2_1: link {
> > > +				clock-lanes = <0>;
> > > +				data-lanes = <2>, <1>;
> > > +				remote = <&imx074_1>;
> > > +			};
> > > +		};
> > > +		port@2 {
> > > +			reg = <2>;			/* port 2: link to the CEU */
> > > +
> > > +			csi2_2: link {
> > > +				immutable;
> > > +				remote = <&ceu0_0>;
> > > +			};
> > > +		};
> > 
> > Maybe the example would be clearer if you split it up in two, one simple
> > case with the csi2_1 <-> imx074_1 and a more advanced with the link in
> > between.
> 
> With no link between two ports no connection is possible, so, only 
> examples with links make sense.

I should have said with the renesas,sh-mobile-ceu in between.

So simple example: csi2_1 <-l-> imx074_1
advanced: csi2_2 <-l-> ceu <-l-> ov772x

> 
> > It took me some time until I figured out that these are two
> > separate camera/sensor pairs. Somehow I was looking for a multiplexer
> > between them.
> 
> Maybe I can add more comments to the file, perhaps, add an ASCII-art 
> chart.

That would be good.

> 
> > I am not sure we really want to have these circular phandles here.
> 
> It has been suggested and accepted during a discussion at the KS / LPC a 
> month ago. The original version only had phandle referencing in one 
> direction.

Ok.

Sascha


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