Re: [PATCH v13 02/10] dt-bindings: document devicetree bindings for mux-controllers and gpio-mux

[Philipp Zabel <p.zabel@pengutronix.de>]

On Wed, 2017-04-19 at 12:41 +0200, Peter Rosin wrote:
> On 2017-04-19 11:17, Philipp Zabel wrote:
> > On Tue, 2017-04-18 at 15:36 +0200, Peter Rosin wrote:
> >> If I got things wrong when I skimmed whatever I came across, and if the
> >> mmio register is the only mux control option in the stars, it becomes
> >> less obvious... It's of course still possible to hook into the mux
> >> subsystem, but the benefit is questionable. And you do get the extra
> >> device tree node. You could of course also implement a mux driver
> >> outside of drivers/mux and thus make use of the mux api, but it's tiny
> >> and any benefit is truly small.
> > 
> > What I wondered mostly is whether it would be a good idea to move the
> > OF-graph ports into the mux controller node, and let the video capture
> > device be the consumer of the mux.
> > But this wouldn't fit well with the clear split between the mux
> > controller and the actual mux hardware in the mux DT bindings.
> 
> I have tried to do something similar. I think. The current
> drivers/i2c/muxes/i2c-mux-gpio.c is a good candidate for the same thing
> IIUC.
>
> That dedicated driver and the general purpose i2c mux driver does pretty
> much the same thing with these two DT snippets:
> 
> Dedicated i2c-mux-gpio DT snippet:
> 
> 	i2c-mux {
> 		compatible = "i2c-mux-gpio";
> 		i2c-parent = <&i2c1>;
> 
> 		mux-gpios = <&gpio1 22 0 &gpio1 23 0>;
> 
> 		#address-cells = <1>;
> 		#size-cells = <0>;
> 
> 		i2c@1 {
> 			...
> 		};
> 
> 		i2c@3 {
> 			...
> 		};
> 	};
> 
> General purpose mux DT snippet:
> 
> 	mux: mux-controller {
> 		compatible = "gpio-mux";
> 		#mux-control-cells = <0>;
> 
> 		mux-gpios = <&gpio1 22 0 &gpio1 23 0>;
> 	};
> 
> 	i2c-mux {
> 		compatible = "i2c-mux";
> 		i2c-parent = <&i2c1>;
> 
> 		mux-controls = <&mux>;
> 
> 		#address-cells = <1>;
> 		#size-cells = <0>;
> 
> 		i2c@1 {
> 			...
> 		};
> 
> 		i2c@3 {
> 			...
> 		};
> 	};

Yes, replace i2c-mux with video-mux and the i2c@x nodes with port@x
nodes, and this is very close to what I am thinking about.

> I would love to find a way to cleanly get the mux framework to handle
> the first DT as well, and thus being able to obsolete the dedicated
> i2c-mux-gpio driver. I have not figured out how to accomplish that
> without abusing the driver-model to a point that it's not working.
> Help with that task is dearly appreciated.
> 
> What I have stumbled on, I think, is that two drivers needs to be
> instantiated from the same DT node. At the same time, I need the
> mux framework to handle the current out-of-node thing with a
> phandle as well, so that several mux consumers can share a common
> mux controller. My understanding of these matters are apparently not
> deep enough...

Not necessarily, if the framework could export a function to create a
gpio/mmio mux_chip on a given device and the gpio-mux and *-mux-gpio
drivers just reuse that.

> I think you would like a DT that looks more like the first DT
> snippet but still enjoy the flexibility of the mux framework and w/o
> implementing a (another) full muxing sub-sub-system like the i2c
> sub-system has done. Correct?

Correct.

regards
Philipp