linux-kernel - Re: [RFC 1/2] dt-bindings: add mmio-based syscon mux controller DT bindings

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Date:   Tue, 18 Apr 2017 13:08:41 +0300
From:   Sakari Ailus <sakari.ailus@....fi>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Peter Rosin <peda@...ntia.se>, Pavel Machek <pavel@....cz>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Steve Longerbeam <slongerbeam@...il.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...gutronix.de
Subject: Re: [RFC 1/2] dt-bindings: add mmio-based syscon mux controller DT
 bindings

Hi Philipp,

On Tue, Apr 18, 2017 at 10:19:04AM +0200, Philipp Zabel wrote:
> On Thu, 2017-04-13 at 17:48 +0200, Philipp Zabel wrote:
> > This adds device tree binding documentation for mmio-based syscon
> > multiplexers controlled by a single bitfield in a syscon register
> > range.
> > 
> > Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
> > ---
> >  Documentation/devicetree/bindings/mux/mmio-mux.txt | 56 ++++++++++++++++++++++
> >  1 file changed, 56 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/mux/mmio-mux.txt
> > 
> > diff --git a/Documentation/devicetree/bindings/mux/mmio-mux.txt b/Documentation/devicetree/bindings/mux/mmio-mux.txt
> > new file mode 100644
> > index 0000000000000..11d96f5d98583
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/mux/mmio-mux.txt
> > @@ -0,0 +1,56 @@
> > +MMIO bitfield-based multiplexer controller bindings
> > +
> > +Define a syscon bitfield to be used to control a multiplexer. The parent
> > +device tree node must be a syscon node to provide register access.
> > +
> > +Required properties:
> > +- compatible : "gpio-mux"
> > +- reg : register base of the register containing the control bitfield
> > +- bit-mask : bitmask of the control bitfield in the control register
> > +- bit-shift : bit offset of the control bitfield in the control register
> > +- #mux-control-cells : <0>
> > +* Standard mux-controller bindings as decribed in mux-controller.txt
> > +
> > +Optional properties:
> > +- idle-state : if present, the state the mux will have when idle. The
> > +	       special state MUX_IDLE_AS_IS is the default.
> > +
> > +The multiplexer state is defined as the value of the bitfield described
> > +by the reg, bit-mask, and bit-shift properties, accessed through the parent
> > +syscon.
> > +
> > +Example:
> > +
> > +	syscon {
> > +		compatible = "syscon";
> > +
> > +		mux: mux-controller@3 {
> > +			compatible = "mmio-mux";
> > +			reg = <0x3>;
> > +			bit-mask = <0x1>;
> > +			bit-shift = <5>;
> > +			#mux-control-cells = <0>;
> > +		};
> > +	};
> > +
> > +	video-mux {
> > +		compatible = "video-mux";
> > +		mux-controls = <&mux>;
> > +
> > +		ports {
> > +			/* input 0 */
> > +			port@0 {
> > +				reg = <0>;
> > +			};
> > +
> > +			/* input 1 */
> > +			port@1 {
> > +				reg = <1>;
> > +			};
> > +
> > +			/* output */
> > +			port@2 {
> > +				reg = <2>;
> > +			};
> > +		};
> > +	};
> 
> So Pavel (added to Cc:) suggested to merge these into one node for the
> video mux, as really we are describing a single hardware entity that
> happens to be multiplexing multiple video buses into one:

Two drivers will be needed in a way or another to disconnect the dependency
between the video switch driver and the MUX implementation. Are there ways
to do that cleanly other than having two devices?

And if there are two devices, shouldn't the video switch device be a child
of the MUX device? I think it'd be odd to have it hanging around in a
completely unrelated part of the device tree.

> 
> 	syscon {
> 		compatible = "syscon";
> 
> 		/* video multiplexer */
> 		mux: mux-controller@3 {
> 			compatible = "video-mmio-mux";
> 			reg = <0x3>;
> 			bit-mask = <0x1>;
> 			bit-shift = <5>;
> 			#mux-control-cells = <0>;
>  
> 			mux-controls = <&mux>;
> 
> 			ports {
> 				/* input 0 */
> 				port@0 {
> 					reg = <0>;
> 				};
> 
> 				/* input 1 */
> 				port@1 {
> 					reg = <1>;
> 				};
> 
> 				/* output */
> 				port@2 {
> 					reg = <2>;
> 				};
> 			};
> 		};
> 	};
> 
> That would not touch on this "general purpose" mmio-mux binding itself,
> but would make it necessary to add a separate "video-mmio-mux" and a
> "video-gpio-mux" binding that mirror the "mmio-mux" and "gpio-mux"
> bindings but add the OF-graph connections.
> 
> Also I think in this case the self-referencing mux-controls property
> would be superfluous, as the driver binding to this node is expected to
> control the mux according to activation of the links described by the
> OF-graph bindings.

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@....fi	XMPP: sailus@...iisi.org.uk