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

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Message-ID: <b2e0c3bb-c74a-3ef3-6b58-1139d7a932a4@axentia.se>
Date:   Thu, 20 Apr 2017 16:13:19 +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 15:32, Peter Rosin wrote:
> 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.

Ok, thinking some more about this. Sorry for spamming and replying to
self...

How about:

 	syscon {
 		compatible = "syscon", "simple-mfd";

		mux: mux-controllers {
			compatible = "mmio-mux";
	 		#mux-control-cells = <1>;

			/* three mux controllers, one at reg 3 bits 0:2,
			 * one at reg 3 bits 5:6 and one at reg 7 bit 3.
			 */
			mux-reg-masks = <0x3 0x07>, <0x3 0x60>, <0x7 0x08>;
			idle-state = <7>, <MUX_IDLE_AS_IS>, <0>;
		};

 
 		video-mux {
 			compatible = "video-mux";
 			mux-controls = <&mux 1>; /* i.e. reg 3 bits 5:6 */
 
 			ports {
 				/* ports 0..5 */
 			};
 		};
 	};

Optionally using some 64-bit safe 3-value encoding of the register fields
in the mux-reg-masks binding...

Cheers,
peda