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 13:51:59 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>,
        Peter Rosin <peda@...ntia.se>,
        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

On Tue 2017-04-18 13:55:40, Sakari Ailus wrote:
> On Tue, Apr 18, 2017 at 12:34:53PM +0200, Pavel Machek wrote:
> > On Tue 2017-04-18 13:08:41, Sakari Ailus wrote:
> > > 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?
> > 
> > Yes.
> > 
> > Device tree describes hardware, not the driver structure.
> 
> I think you you could view the MUX control as a device, too, and that's
> separate from the actual video switch.

Actually, I believe what matters here is hardware. There's some chip,
somewhere, that does the switching, and the device tree should should
basically describe that switch.


									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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