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

[Peter Rosin]

On 2017-04-20 10:14, Philipp Zabel wrote:
> Hi Rob,
> 
> On Wed, 2017-04-19 at 17:09 -0500, 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@xxxxxxxxxxxxxx>
>>> ---
>>>  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.
> 
> Thanks, I have been a bit uneasy with the separate per-bitfield mux
> controller node, so I'm eager to agree. But thit makes me unsure how to
> best represent the information that is spelled out above.
> 
>> I think the parent should be declared as a mux controller instead.
> 
> The syscon node itself should be the mux controller? Would you expect
> the mmio-mux driver bind to the syscon node, or should the mux framework
> handle creation of the mux controls in this case (i.e. does the syscon
> node get a "mmio-mux" added to its compatible list)?
> 
>> You could encode the mux addr and bit position in the mux cells.
> 
> What about the bit-mask / bitfield width? Just add a cell for it?
> 
> 	gpr: syscon {
> 		compatible = "mmio-mux", "syscon", "simple-mfd";
> 		#mux-control-cells = <3>;
> 
> 		video-mux {
> 			compatible = "video-mux";
> 			/* register 0x3, bits [6:5] */
> 			mux-controls = <&gpr 0x3 5 0x3>;
> 
> 			ports {
> 				/* ports 0..5 */
> 			};
> 		};
> 	};
> 
> Or maybe using MSB and LSB would be better to read?
> 
> 		video-mux {
> 			/* register 0x3, bits [6:5] */
> 			mux-control = <&gpr 0x3 6 5>;
> 
> 			ports {
> 				/* ports 0..5 */
> 			};
> 		};

Why do you need three values for one register+field? The shift can be
implied from the mask, if the mask is pre-shifted. I.e. specifying a
mask of 0x60 in this case. What I'm I missing?

>>> +			#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.
> 
> What I especially like about the mux-controls property is that would
> allow me to use the gpio-mux driver (or any other mux controller)
> instead of having to code variants of the video-mux for all possible
> control schemes.

Yes, when talking about e.g. PWMs or GPIOs, there seem to be little
question that they can sit in their own nodes. I don't get the reluctance
to have MUXes in nodes of their own?

PWMs and GPIOs are controlled from one end and mostly used for something
completely different. Just like MUXes. At least there are many MUXes like
that. Agreed, there are also many MUXes where the MUX is tightly integrated
in some bigger function. But in this case, it appears that there is benefit
in moving the MUX to its own DT node and have the MUX consumer oblivious
of the details of the MUX. Why is that a problem?

Cheers,
peda