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

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Date:   Thu, 20 Apr 2017 15:32:21 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Rob Herring <robh@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>
CC:     Mark Rutland <mark.rutland@....com>,
        Sakari Ailus <sakari.ailus@....fi>,
        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 2017-04-20 00:09, Rob Herring wrote:
> On Thu, Apr 13, 2017 at 05:48:11PM +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>;
> 
> This pattern doesn't scale once you have multiple fields @ addr 3. I 
> also don't really think a node per register field in DT really scales.
> 
> I think the parent should be declared as a mux controller instead. You 
> could encode the mux addr and bit position in the mux cells.

But then you need to create mux controllers on demand. I have not
succeeded in doing that while also following the rules of the driver
model. I had severe problems with life-time issues when I tried.
I would like to see code before embarking on this path, and I'm
apparently not the one writing it...

So, either you meant that, or that the parent node should somehow
specify the possible mux controllers up front so that they can be
pre-created and ready when the consumers request them. But if you
do that, you can just refer to them by some enumeration from the
mux consumers instead of by some convoluted reg+field notation.

>> +			#mux-control-cells = <0>;
>> +		};
>> +	};
>> +
>> +	video-mux {
>> +		compatible = "video-mux";
>> +		mux-controls = <&mux>;
> 
> The mux binding was largely defined for a single control controling 
> multiple muxes. This doesn't really fit that, but I guess this is an 
> improvement over a custom syscon phandle.
> 
>> +
>> +		ports {
>> +			/* input 0 */
>> +			port@0 {
>> +				reg = <0>;
>> +			};
>> +
>> +			/* input 1 */
>> +			port@1 {
>> +				reg = <1>;
>> +			};
>> +
>> +			/* output */
>> +			port@2 {
>> +				reg = <2>;
>> +			};
>> +		};
>> +	};
>> -- 
>> 2.11.0
>>