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Re: [PATCH v13 02/10] dt-bindings: document devicetree bindings for mux-controllers and gpio-mux

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  Re: [PATCH v13 02/10] dt-bindings: document devicetree bindings for  mux-controllers and gpio-mux Philipp Zabel <p.zabel@pengutronix.de> - 2017-04-18 12:10 +0200
    Re: [PATCH v13 02/10] dt-bindings: document devicetree bindings for  mux-controllers and gpio-mux Philipp Zabel <p.zabel@pengutronix.de> - 2017-04-19 11:20 +0200
      Re: [PATCH v13 02/10] dt-bindings: document devicetree bindings for  mux-controllers and gpio-mux Philipp Zabel <p.zabel@pengutronix.de> - 2017-04-19 13:10 +0200
        Re: [PATCH v13 02/10] dt-bindings: document devicetree bindings for  mux-controllers and gpio-mux Philipp Zabel <p.zabel@pengutronix.de> - 2017-04-19 18:40 +0200

#1625205 — Re: [PATCH v13 02/10] dt-bindings: document devicetree bindings for mux-controllers and gpio-mux

On Thu, 2017-04-13 at 18:43 +0200, Peter Rosin wrote:
> Allow specifying that a single multiplexer controller can be used to
> control several parallel multiplexers, thus enabling sharing of the
> multiplexer controller by different consumers.
> 
> Add a binding for a first mux controller in the form of a GPIO based mux
> controller.
> 
> Acked-by: Jonathan Cameron <jic23@kernel.org>
> Acked-by: Rob Herring <robh@kernel.org>
> Signed-off-by: Peter Rosin <peda@axentia.se>
> ---
>  Documentation/devicetree/bindings/mux/gpio-mux.txt |  69 +++++++++
>  .../devicetree/bindings/mux/mux-controller.txt     | 157 +++++++++++++++++++++
>  MAINTAINERS                                        |   6 +
>  include/dt-bindings/mux/mux.h                      |  16 +++
>  4 files changed, 248 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/mux/gpio-mux.txt
>  create mode 100644 Documentation/devicetree/bindings/mux/mux-controller.txt
>  create mode 100644 include/dt-bindings/mux/mux.h
> 
> diff --git a/Documentation/devicetree/bindings/mux/gpio-mux.txt b/Documentation/devicetree/bindings/mux/gpio-mux.txt
> new file mode 100644
> index 000000000000..b8f746344d80
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/mux/gpio-mux.txt
> @@ -0,0 +1,69 @@
> +GPIO-based multiplexer controller bindings
> +
> +Define what GPIO pins are used to control a multiplexer. Or several
> +multiplexers, if the same pins control more than one multiplexer.
> +
> +Required properties:
> +- compatible : "gpio-mux"
> +- mux-gpios : list of gpios used to control the multiplexer, least
> +	      significant bit first.
> +- #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 number represented by the
> +multiplexer GPIO pins, where the first pin is the least significant
> +bit. An active pin is a binary 1, an inactive pin is a binary 0.
> +
> +Example:
> +
> +	mux: mux-controller {
> +		compatible = "gpio-mux";
> +		#mux-control-cells = <0>;
> +
> +		mux-gpios = <&pioA 0 GPIO_ACTIVE_HIGH>,
> +			    <&pioA 1 GPIO_ACTIVE_HIGH>;
> +	};
> +
> +	adc-mux {
> +		compatible = "io-channel-mux";
> +		io-channels = <&adc 0>;
> +		io-channel-names = "parent";
> +
> +		mux-controls = <&mux>;
> +
> +		channels = "sync-1", "in", "out", "sync-2";
> +	};

Could you explain in more detail the reasoning behind this split between
the mux controller and the actual mux?
For SoC internal video bus muxes that are controlled by a register
bitfield, it seems a bit strange to have to split them into two device
tree nodes.

Basically I'm trying to figure out whether a video mux (which has a mux
control plus OF-graph bindings to describe its ports and connections)
would fit into the same category as an adc-mux or i2c-mux, or whether it
would be better to handle them as a specialized form of mux-controller.

regards
Philipp

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#1625967

On Tue, 2017-04-18 at 15:36 +0200, Peter Rosin wrote:
> On 2017-04-18 12:06, Philipp Zabel wrote:
> > On Thu, 2017-04-13 at 18:43 +0200, Peter Rosin wrote:
> >> Allow specifying that a single multiplexer controller can be used to
> >> control several parallel multiplexers, thus enabling sharing of the
> >> multiplexer controller by different consumers.
> >>
> >> Add a binding for a first mux controller in the form of a GPIO based mux
> >> controller.
> >>
> >> Acked-by: Jonathan Cameron <jic23@kernel.org>
> >> Acked-by: Rob Herring <robh@kernel.org>
> >> Signed-off-by: Peter Rosin <peda@axentia.se>
> >> ---
> >>  Documentation/devicetree/bindings/mux/gpio-mux.txt |  69 +++++++++
> >>  .../devicetree/bindings/mux/mux-controller.txt     | 157 +++++++++++++++++++++
> >>  MAINTAINERS                                        |   6 +
> >>  include/dt-bindings/mux/mux.h                      |  16 +++
> >>  4 files changed, 248 insertions(+)
> >>  create mode 100644 Documentation/devicetree/bindings/mux/gpio-mux.txt
> >>  create mode 100644 Documentation/devicetree/bindings/mux/mux-controller.txt
> >>  create mode 100644 include/dt-bindings/mux/mux.h
> >>
> >> diff --git a/Documentation/devicetree/bindings/mux/gpio-mux.txt b/Documentation/devicetree/bindings/mux/gpio-mux.txt
> >> new file mode 100644
> >> index 000000000000..b8f746344d80
> >> --- /dev/null
> >> +++ b/Documentation/devicetree/bindings/mux/gpio-mux.txt
> >> @@ -0,0 +1,69 @@
> >> +GPIO-based multiplexer controller bindings
> >> +
> >> +Define what GPIO pins are used to control a multiplexer. Or several
> >> +multiplexers, if the same pins control more than one multiplexer.
> >> +
> >> +Required properties:
> >> +- compatible : "gpio-mux"
> >> +- mux-gpios : list of gpios used to control the multiplexer, least
> >> +	      significant bit first.
> >> +- #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 number represented by the
> >> +multiplexer GPIO pins, where the first pin is the least significant
> >> +bit. An active pin is a binary 1, an inactive pin is a binary 0.
> >> +
> >> +Example:
> >> +
> >> +	mux: mux-controller {
> >> +		compatible = "gpio-mux";
> >> +		#mux-control-cells = <0>;
> >> +
> >> +		mux-gpios = <&pioA 0 GPIO_ACTIVE_HIGH>,
> >> +			    <&pioA 1 GPIO_ACTIVE_HIGH>;
> >> +	};
> >> +
> >> +	adc-mux {
> >> +		compatible = "io-channel-mux";
> >> +		io-channels = <&adc 0>;
> >> +		io-channel-names = "parent";
> >> +
> >> +		mux-controls = <&mux>;
> >> +
> >> +		channels = "sync-1", "in", "out", "sync-2";
> >> +	};
> > 
> > Could you explain in more detail the reasoning behind this split between
> > the mux controller and the actual mux?
> > For SoC internal video bus muxes that are controlled by a register
> > bitfield, it seems a bit strange to have to split them into two device
> > tree nodes.
> 
> The background for the split is in the cover letter.

Thanks for explaining anyway, I didn't read past the changelog earlier.

[...]
> > Basically I'm trying to figure out whether a video mux (which has a mux
> > control plus OF-graph bindings to describe its ports and connections)
> > would fit into the same category as an adc-mux or i2c-mux, or whether it
> > would be better to handle them as a specialized form of mux-controller.
> 
> I did read some earlier thread about your muxing requirements and I got
> the impression that you also had HW which controlled the mux with
> gpio lines? In that case, the mux subsystem seems like a perfect fit
> with a new syscon/mmio/reg based mux driver (or whatever the name should
> be, I think I'd go with syscon) pretty much as suggested in your RFC
> patches. And then of course reuse the existing gpio-mux driver for the
> other case.

Yes, the requirement on hand is for MMIO controlled SoC internal muxes
for the i.MX6 video capture subsystem, but I'd like to also support GPIO
controlled external muxes to switch between two camera sources on those
boards that have them.

> The video-mux would fit as a mux consumer just like the iio-mux and the
> i2c-mux are mux consumers, with input 0/input 1 being the port that
> would be selected with the mux I guess.

Exactly. An N-input mux would have N+1 ports with port N being the
output.

[...]
> If I got things wrong when I skimmed whatever I came across, and if the
> mmio register is the only mux control option in the stars, it becomes
> less obvious... It's of course still possible to hook into the mux
> subsystem, but the benefit is questionable. And you do get the extra
> device tree node. You could of course also implement a mux driver
> outside of drivers/mux and thus make use of the mux api, but it's tiny
> and any benefit is truly small.

What I wondered mostly is whether it would be a good idea to move the
OF-graph ports into the mux controller node, and let the video capture
device be the consumer of the mux.
But this wouldn't fit well with the clear split between the mux
controller and the actual mux hardware in the mux DT bindings.

regards
Philipp

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#1626026

On Wed, 2017-04-19 at 12:41 +0200, Peter Rosin wrote:
> On 2017-04-19 11:17, Philipp Zabel wrote:
> > On Tue, 2017-04-18 at 15:36 +0200, Peter Rosin wrote:
> >> If I got things wrong when I skimmed whatever I came across, and if the
> >> mmio register is the only mux control option in the stars, it becomes
> >> less obvious... It's of course still possible to hook into the mux
> >> subsystem, but the benefit is questionable. And you do get the extra
> >> device tree node. You could of course also implement a mux driver
> >> outside of drivers/mux and thus make use of the mux api, but it's tiny
> >> and any benefit is truly small.
> > 
> > What I wondered mostly is whether it would be a good idea to move the
> > OF-graph ports into the mux controller node, and let the video capture
> > device be the consumer of the mux.
> > But this wouldn't fit well with the clear split between the mux
> > controller and the actual mux hardware in the mux DT bindings.
> 
> I have tried to do something similar. I think. The current
> drivers/i2c/muxes/i2c-mux-gpio.c is a good candidate for the same thing
> IIUC.
>
> That dedicated driver and the general purpose i2c mux driver does pretty
> much the same thing with these two DT snippets:
> 
> Dedicated i2c-mux-gpio DT snippet:
> 
> 	i2c-mux {
> 		compatible = "i2c-mux-gpio";
> 		i2c-parent = <&i2c1>;
> 
> 		mux-gpios = <&gpio1 22 0 &gpio1 23 0>;
> 
> 		#address-cells = <1>;
> 		#size-cells = <0>;
> 
> 		i2c@1 {
> 			...
> 		};
> 
> 		i2c@3 {
> 			...
> 		};
> 	};
> 
> General purpose mux DT snippet:
> 
> 	mux: mux-controller {
> 		compatible = "gpio-mux";
> 		#mux-control-cells = <0>;
> 
> 		mux-gpios = <&gpio1 22 0 &gpio1 23 0>;
> 	};
> 
> 	i2c-mux {
> 		compatible = "i2c-mux";
> 		i2c-parent = <&i2c1>;
> 
> 		mux-controls = <&mux>;
> 
> 		#address-cells = <1>;
> 		#size-cells = <0>;
> 
> 		i2c@1 {
> 			...
> 		};
> 
> 		i2c@3 {
> 			...
> 		};
> 	};

Yes, replace i2c-mux with video-mux and the i2c@x nodes with port@x
nodes, and this is very close to what I am thinking about.

> I would love to find a way to cleanly get the mux framework to handle
> the first DT as well, and thus being able to obsolete the dedicated
> i2c-mux-gpio driver. I have not figured out how to accomplish that
> without abusing the driver-model to a point that it's not working.
> Help with that task is dearly appreciated.
> 
> What I have stumbled on, I think, is that two drivers needs to be
> instantiated from the same DT node. At the same time, I need the
> mux framework to handle the current out-of-node thing with a
> phandle as well, so that several mux consumers can share a common
> mux controller. My understanding of these matters are apparently not
> deep enough...

Not necessarily, if the framework could export a function to create a
gpio/mmio mux_chip on a given device and the gpio-mux and *-mux-gpio
drivers just reuse that.

> I think you would like a DT that looks more like the first DT
> snippet but still enjoy the flexibility of the mux framework and w/o
> implementing a (another) full muxing sub-sub-system like the i2c
> sub-system has done. Correct?

Correct.

regards
Philipp

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#1626493

On Wed, 2017-04-19 at 13:23 +0200, Peter Rosin wrote:
> On 2017-04-19 13:05, Philipp Zabel wrote:
> > On Wed, 2017-04-19 at 12:41 +0200, Peter Rosin wrote:
> >> On 2017-04-19 11:17, Philipp Zabel wrote:
> >>> On Tue, 2017-04-18 at 15:36 +0200, Peter Rosin wrote:
> >>>> If I got things wrong when I skimmed whatever I came across, and if the
> >>>> mmio register is the only mux control option in the stars, it becomes
> >>>> less obvious... It's of course still possible to hook into the mux
> >>>> subsystem, but the benefit is questionable. And you do get the extra
> >>>> device tree node. You could of course also implement a mux driver
> >>>> outside of drivers/mux and thus make use of the mux api, but it's tiny
> >>>> and any benefit is truly small.
> >>>
> >>> What I wondered mostly is whether it would be a good idea to move the
> >>> OF-graph ports into the mux controller node, and let the video capture
> >>> device be the consumer of the mux.
> >>> But this wouldn't fit well with the clear split between the mux
> >>> controller and the actual mux hardware in the mux DT bindings.
> >>
> >> I have tried to do something similar. I think. The current
> >> drivers/i2c/muxes/i2c-mux-gpio.c is a good candidate for the same thing
> >> IIUC.
> >>
> >> That dedicated driver and the general purpose i2c mux driver does pretty
> >> much the same thing with these two DT snippets:
> >>
> >> Dedicated i2c-mux-gpio DT snippet:
> >>
> >> 	i2c-mux {
> >> 		compatible = "i2c-mux-gpio";
> >> 		i2c-parent = <&i2c1>;
> >>
> >> 		mux-gpios = <&gpio1 22 0 &gpio1 23 0>;
> >>
> >> 		#address-cells = <1>;
> >> 		#size-cells = <0>;
> >>
> >> 		i2c@1 {
> >> 			...
> >> 		};
> >>
> >> 		i2c@3 {
> >> 			...
> >> 		};
> >> 	};
> >>
> >> General purpose mux DT snippet:
> >>
> >> 	mux: mux-controller {
> >> 		compatible = "gpio-mux";
> >> 		#mux-control-cells = <0>;
> >>
> >> 		mux-gpios = <&gpio1 22 0 &gpio1 23 0>;
> >> 	};
> >>
> >> 	i2c-mux {
> >> 		compatible = "i2c-mux";
> >> 		i2c-parent = <&i2c1>;
> >>
> >> 		mux-controls = <&mux>;
> >>
> >> 		#address-cells = <1>;
> >> 		#size-cells = <0>;
> >>
> >> 		i2c@1 {
> >> 			...
> >> 		};
> >>
> >> 		i2c@3 {
> >> 			...
> >> 		};
> >> 	};
> > 
> > Yes, replace i2c-mux with video-mux and the i2c@x nodes with port@x
> > nodes, and this is very close to what I am thinking about.
> > 
> >> I would love to find a way to cleanly get the mux framework to handle
> >> the first DT as well, and thus being able to obsolete the dedicated
> >> i2c-mux-gpio driver. I have not figured out how to accomplish that
> >> without abusing the driver-model to a point that it's not working.
> >> Help with that task is dearly appreciated.
> >>
> >> What I have stumbled on, I think, is that two drivers needs to be
> >> instantiated from the same DT node. At the same time, I need the
> >> mux framework to handle the current out-of-node thing with a
> >> phandle as well, so that several mux consumers can share a common
> >> mux controller. My understanding of these matters are apparently not
> >> deep enough...
> > 
> > Not necessarily, if the framework could export a function to create a
> > gpio/mmio mux_chip on a given device and the gpio-mux and *-mux-gpio
> > drivers just reuse that.
> 
> I've been up that creek. Why should the gpio mux be special cased?

You are right, this does not scale.

> That's not clean, the implication is that all mux consumers need
> to handle the gpio case and have a special compatible for that
> case etc. Then someone thinks the DT should look equally "clean" for
> some i2c based mux, and the weeds start piling up. This is exactly
> what we don't want. We want the mux consumer drivers to be totally
> agnostic about the fact that they happen to use a gpio mux.

If you want to have i2c-mux-gpio and i2c-mux compatibles, and a single
driver to handle them both, it must at least match both compatibles, so
it can't be completely agnostic.

Why not then have it call

	if (/* compatible == "i2c-mux" */)
		mux = devm_mux_control_get(dev, NULL);
	else /* if (compatible == "i2c-mux-gpio/mmio/etc.") */
		mux = devm_mux_control_create(dev);

? The mux framework core could hold a list of those <usage>-mux-<type>
compatibles and dispatch creation of the correct mux (or mux platform
device, if necessary).

regards
Philipp

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