[RFC] media DT bindings

[Guennadi Liakhovetski]

On Tue, 31 Jul 2012, Laurent Pinchart wrote:

> Hi Guennadi,
> 
> On Tuesday 31 July 2012 11:26:27 Guennadi Liakhovetski wrote:
> > On Fri, 27 Jul 2012, Laurent Pinchart wrote:
> > > Hi Sylwester,
> > > 
> > > On Wednesday 18 July 2012 19:00:15 Sylwester Nawrocki wrote:
> > > > On 07/16/2012 01:41 PM, Guennadi Liakhovetski wrote:
> > [snip]
> > 
> > > > >>> An sh-mobile CEU DT node could look like
> > > > >>> 
> > > > >>> 	ceu0 at 0xfe910000 = {
> > > > >>> 	
> > > > >>> 		compatible = "renesas,sh-mobile-ceu";
> > > > >>> 		reg =<0xfe910000 0xa0>;
> > > > >>> 		interrupts =<0x880>;
> > > > >>> 		bus-width =<16>;		/* #lines routed on the board */
> > > > >>> 		clock-frequency =<50000000>;	/* max clock */
> > > > >>> 		#address-cells =<1>;
> > > > >>> 		#size-cells =<0>;
> > > > >>> 		...
> > > > >>> 		ov772x-1 = {
> > > > >>> 		
> > > > >>> 			reg =<0>;
> > > > 
> > > > This property might be redundant, we already have the "client" phandle
> > > > pointing to "ov772x at 0x21-0", which has all interesting properties inside
> > > > it. Other than there is probably no reasonable usage for it under
> > > > "ceu0 at 0xfe910000" node ?
> > > > 
> > > > >>> 			client =<&ov772x at 0x21-0>;
> > > > >>> 			local-pad = "parallel-sink";
> > > > >>> 			remote-pad = "parallel-source";
> > > > >> 
> > > > >> I'm not sure I like that. Is it really needed when we already have
> > > > >> the child/parent properties around ?
> > > > > 
> > > > > I think it is. Both the host and the client can have multiple pads
> > > > > (e.g.,
> > > > > parallel / serial). These properties specify which pads are used and
> > > > > make
> > > > > the translation between DT data and our subdev / pad APIs simpler.
> > > > 
> > > > OK, sorry, but isn't it all about just specifying what sort of data bus
> > > > is used ? :-)
> > > 
> > > In some (many/most ?) cases probably, but not in all of them.
> > > 
> > > What about merging the client and remote-pad properties ? The resulting
> > > property would then reference a pad with <&ov772x at 0x21-0 0>.
> > 
> > What would the "0" parameter mean then? Pad #0?
> 
> Yes.
> 
> > But aren't these numbers device specific? Maybe not a huge deal, but these
> > numbers are defind by the driver, right? Not the DT itself. So, drivers then
> > will have to take care not to change their pad numbering. Whereas using
> > strings, we can fix strings in the common V4L DT spec and keep them standard
> > across devices and drivers. Then drivers might be less likely to change
> > these assignments randomly ;-)
> 
> Userspace applications usually rely on pad numbers as well, so I consider them 
> as more or less part of the ABI. If we really need to, we could add a DT pad 
> number -> media controller pad number conversion in the driver, that would be 
> less expensive than pad name -> pad number conversion (especially since it 
> would be skipped in most cases).

Ok, then, how about

		#address-cells = <1>;
		#size-cells = <0>;
		...
		ov772x-1 = {
			reg = <1>;			/* local pad # */
			client = <&ov772x at 0x21-0 0>;	/* remote phandle and pad */


Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/