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Re: [PATCH] dt-bindings: i2c: add bindings for nxp,pca9541

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  Re: [PATCH] dt-bindings: i2c: add bindings for nxp,pca9541 Guenter Roeck <linux@roeck-us.net> - 2016-06-27 15:20 +0200
    Re: [PATCH] dt-bindings: i2c: add bindings for nxp,pca9541 Rob Herring <robh@kernel.org> - 2016-07-01 03:40 +0200
      Re: [PATCH] dt-bindings: i2c: add bindings for nxp,pca9541 Guenter Roeck <linux@roeck-us.net> - 2016-07-06 17:20 +0200

#1432035 — Re: [PATCH] dt-bindings: i2c: add bindings for nxp,pca9541

On 06/27/2016 03:11 AM, Peter Rosin wrote:
> Fill the gap for this pre-existing driver.
>
> Signed-off-by: Peter Rosin <peda@axentia.se>
> ---
>   .../devicetree/bindings/i2c/i2c-arb-pca9541.txt    | 33 ++++++++++++++++++++++
>   MAINTAINERS                                        |  1 +
>   2 files changed, 34 insertions(+)
>   create mode 100644 Documentation/devicetree/bindings/i2c/i2c-arb-pca9541.txt
>
> Hi!
>
> I'm wondering about this driver. It is not a trivial device, and yet it
> has historically relied on the i2c core matching the chip w/o vendor
> prefix. This is not ideal. But what to do about the driver implementing
> this in terms of an i2c-mux, somthing which the chip is not; It is an
> i2c arbitrator. It just happens to rely on the i2c mux core also handling
> i2c gates and i2c arbitrators. But that seems like a Linux detail. So I
> don't know what to do here?
>

The concept of arbitrators didn't exist when I wrote the driver. I would not
have a problem with renaming the file if that is what you are asking for.

> That is, the patch - as is - describes something that would be trivial to
> support today, but at the same time it seems to be too tied to Linux.
>
> The problem is that the i2c@0 intermediate node is not really needed, but
> at the same time removing it would cause a disruption for the driver since
> it can't really use the i2c mux core if that node isn't there. I don't
> see a simple way to fix that in the i2c mux core either (but admittedly
> haven't given it too much thought).
>

The gpio arbitrator uses the same principle as well. Why not just leave it
alone ? Besides, I think it is a good idea to have it, since it groups
the i2c devices behind the chip together. I would not consider that to be
a Linuxism, but a design choice.

Guenter

> Any suggestions?
>
> Cheers,
> Peter
>
> PS. The driver source is in drivers/i2c/muxes/i2c-mux-pca9541.c
>
> diff --git a/Documentation/devicetree/bindings/i2c/i2c-arb-pca9541.txt b/Documentation/devicetree/bindings/i2c/i2c-arb-pca9541.txt
> new file mode 100644
> index 000000000000..edbe84935906
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/i2c/i2c-arb-pca9541.txt
> @@ -0,0 +1,33 @@
> +* NXP PCA9541 I2C bus master selector
> +
> +Required Properties:
> +
> +  - compatible: Must be "nxp,pca9541"
> +
> +  - reg: The I2C address of the device.
> +
> +  The following required properties are defined externally:
> +
> +  - Standard I2C mux properties. See i2c-mux.txt in this directory.
> +  - I2C child bus nodes. See i2c-mux.txt in this directory.
> +
> +
> +Example:
> +
> +	i2c-arbitrator@74 {
> +		compatible = "nxp,pca9541";
> +		#address-cells = <1>;
> +		#size-cells = <0>;
> +		reg = <0x74>;
> +
> +		i2c@0 {
> +			#address-cells = <1>;
> +			#size-cells = <0>;
> +			reg = <0>;
> +
> +			eeprom@54 {
> +				compatible = "at,24c08";
> +				reg = <0x54>;
> +			};
> +		};
> +	};
> diff --git a/MAINTAINERS b/MAINTAINERS
> index e1b090f86e0d..3dd44d0d166c 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -5521,6 +5521,7 @@ S:	Maintained
>   F:	Documentation/i2c/i2c-topology
>   F:	Documentation/i2c/muxes/
>   F:	Documentation/devicetree/bindings/i2c/i2c-mux*
> +F:	Documentation/devicetree/bindings/i2c/i2c-arb*
>   F:	drivers/i2c/i2c-mux.c
>   F:	drivers/i2c/muxes/
>   F:	include/linux/i2c-mux.h
>

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#1434952

On Mon, Jun 27, 2016 at 06:27:21PM +0200, Peter Rosin wrote:
> On 2016-06-27 15:17, Guenter Roeck wrote:
> > On 06/27/2016 03:11 AM, Peter Rosin wrote:
> >> Fill the gap for this pre-existing driver.
> >>
> >> Signed-off-by: Peter Rosin <peda@axentia.se>
> >> ---
> >>   .../devicetree/bindings/i2c/i2c-arb-pca9541.txt    | 33 ++++++++++++++++++++++
> >>   MAINTAINERS                                        |  1 +
> >>   2 files changed, 34 insertions(+)
> >>   create mode 100644 Documentation/devicetree/bindings/i2c/i2c-arb-pca9541.txt
> >>
> >> Hi!
> >>
> >> I'm wondering about this driver. It is not a trivial device, and yet it
> >> has historically relied on the i2c core matching the chip w/o vendor
> >> prefix. This is not ideal. But what to do about the driver implementing
> >> this in terms of an i2c-mux, somthing which the chip is not; It is an
> >> i2c arbitrator. It just happens to rely on the i2c mux core also handling
> >> i2c gates and i2c arbitrators. But that seems like a Linux detail. So I
> >> don't know what to do here?
> >>
> > 
> > The concept of arbitrators didn't exist when I wrote the driver. I would not
> > have a problem with renaming the file if that is what you are asking for.
> 
> No, that was not my issue, I just wanted to document bindings for pca9541,
> and I didn't like how it turned out.
> 
> I don't really care if the bindings doc is named i2c-mux-pca9541.txt (that
> would match the name of the driver, but it still wouldn't make the chip a mux).

So name it i2c-pca9541.txt or the somewhat standard nxp,pca9541.txt 
following the compatible.

> 
> >> That is, the patch - as is - describes something that would be trivial to
> >> support today, but at the same time it seems to be too tied to Linux.
> >>
> >> The problem is that the i2c@0 intermediate node is not really needed, but
> >> at the same time removing it would cause a disruption for the driver since
> >> it can't really use the i2c mux core if that node isn't there. I don't
> >> see a simple way to fix that in the i2c mux core either (but admittedly
> >> haven't given it too much thought).
> >>
> > 
> > The gpio arbitrator uses the same principle as well. Why not just leave it
> > alone ? Besides, I think it is a good idea to have it, since it groups
> > the i2c devices behind the chip together. I would not consider that to be
> > a Linuxism, but a design choice.
> 
> The grouping argument would make sense if there was anything outside the
> group. Also, the required reg property and the extra #address-cells and
> #size-cells doesn't add anything and just gets in the way, and is indeed
> the result of Linuxisms leaking back into device trees.
> 
> If there were no muxes and this was a new driver, the example bindings
> would almost certainly have been something like:
> 
> 	i2c-arbitrator@74 {
> 		compatible = "nxp,pca9541";
> 		reg = <0x74>;
> 
> 		#address-cells = <1>;
> 		#size-cells = <0>;
> 
> 		eeprom@54 {
> 			compatible = "at,24c08";
> 			reg = <0x54>;
> 		};
> 	};
> 
> which I find much nicer.

Yes.

> But, I can't find a way to implement that and keep backwards compatibility
> with old existing device trees.

I don't see any in the kernel tree nor is it documented, so there is not 
compatibility to worry about.

> 
> Which is why I submitted the patch I did. It documents the pca9541 bindings,
> something which is lacking, in terms of i2c-mux as the driver is written.
> At the same time, this feels ugly and exposes linuxism and I wanted to make
> that clear up front. The above simply looks better than the example in the
> patch.
> 
> I intended to mark the submission [RFC PATCH], but I now realize that that
> went missing along the way, sorry.

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#1437792

On 07/06/2016 03:12 AM, Peter Rosin wrote:
> On 2016-07-01 03:20, Rob Herring wrote:
>> On Mon, Jun 27, 2016 at 06:27:21PM +0200, Peter Rosin wrote:
>>> On 2016-06-27 15:17, Guenter Roeck wrote:
>>>> On 06/27/2016 03:11 AM, Peter Rosin wrote:
>>>>> Fill the gap for this pre-existing driver.
>>>>>
>>>>> Signed-off-by: Peter Rosin <peda@axentia.se>
>>>>> ---
>>>>>    .../devicetree/bindings/i2c/i2c-arb-pca9541.txt    | 33 ++++++++++++++++++++++
>>>>>    MAINTAINERS                                        |  1 +
>>>>>    2 files changed, 34 insertions(+)
>>>>>    create mode 100644 Documentation/devicetree/bindings/i2c/i2c-arb-pca9541.txt
>>>>>
>>>>> Hi!
>>>>>
>>>>> I'm wondering about this driver. It is not a trivial device, and yet it
>>>>> has historically relied on the i2c core matching the chip w/o vendor
>>>>> prefix. This is not ideal. But what to do about the driver implementing
>>>>> this in terms of an i2c-mux, somthing which the chip is not; It is an
>>>>> i2c arbitrator. It just happens to rely on the i2c mux core also handling
>>>>> i2c gates and i2c arbitrators. But that seems like a Linux detail. So I
>>>>> don't know what to do here?
>>>>>
>>>>
>>>> The concept of arbitrators didn't exist when I wrote the driver. I would not
>>>> have a problem with renaming the file if that is what you are asking for.
>>>
>>> No, that was not my issue, I just wanted to document bindings for pca9541,
>>> and I didn't like how it turned out.
>>>
>>> I don't really care if the bindings doc is named i2c-mux-pca9541.txt (that
>>> would match the name of the driver, but it still wouldn't make the chip a mux).
>>
>> So name it i2c-pca9541.txt or the somewhat standard nxp,pca9541.txt
>> following the compatible.
>>
>>>
>>>>> That is, the patch - as is - describes something that would be trivial to
>>>>> support today, but at the same time it seems to be too tied to Linux.
>>>>>
>>>>> The problem is that the i2c@0 intermediate node is not really needed, but
>>>>> at the same time removing it would cause a disruption for the driver since
>>>>> it can't really use the i2c mux core if that node isn't there. I don't
>>>>> see a simple way to fix that in the i2c mux core either (but admittedly
>>>>> haven't given it too much thought).
>>>>>
>>>>
>>>> The gpio arbitrator uses the same principle as well. Why not just leave it
>>>> alone ? Besides, I think it is a good idea to have it, since it groups
>>>> the i2c devices behind the chip together. I would not consider that to be
>>>> a Linuxism, but a design choice.
>>>
>>> The grouping argument would make sense if there was anything outside the
>>> group. Also, the required reg property and the extra #address-cells and
>>> #size-cells doesn't add anything and just gets in the way, and is indeed
>>> the result of Linuxisms leaking back into device trees.
>>>
>>> If there were no muxes and this was a new driver, the example bindings
>>> would almost certainly have been something like:
>>>
>>> 	i2c-arbitrator@74 {
>>> 		compatible = "nxp,pca9541";
>>> 		reg = <0x74>;
>>>
>>> 		#address-cells = <1>;
>>> 		#size-cells = <0>;
>>>
>>> 		eeprom@54 {
>>> 			compatible = "at,24c08";
>>> 			reg = <0x54>;
>>> 		};
>>> 	};
>>>
>>> which I find much nicer.
>>
>> Yes.
>>
>>> But, I can't find a way to implement that and keep backwards compatibility
>>> with old existing device trees.
>>
>> I don't see any in the kernel tree nor is it documented, so there is not
>> compatibility to worry about.
>
> Why do you not care about pre-existing device trees not submitted
> to mainline? Is there some statement that DTs are not covered by the
> no-regressions-rule?
>
> So, if I instead had submitted the device tree for my boring
> one-off-ish hardware that few people will ever use, which uses the
> currently working (i.e. as written in my patch) syntax of configuring
> the pca9541 in a device tree, then there would be a "user", things
> would be set in stone and the DT patch as proposed would be
> acceptable?
>
> That is just silly, as I assume you do not want the churn of the
> device trees for all kinds of strange one-off devices? Or do you?
>
> We also have to consider the fact that Guenter (who authored the
> driver) thinks it's a design choice to have the extra DT level...
>

I don't see the point, I think it hurts readability, and I preferred
to have i2c properties clearly separated from arbiter properties.
Given that the current properties are not broken, I think it is just
a change for the sake of a change. I dislike the notion that changes for
the sake of changes are ok as long as there are no in-kernel uses (after all,
this can go both ways). In short, I don't like it, but then I don't have
to like or approve it either, so that doesn't mean much.

I assume this will be changed for all arbiters, to have a consistent set
of bindings for the same type of devices ? Or will i2c-arb-gpio-challenge
be unmodified since it _does_ have an in-kernel users, and it will be up
to each arbiter to define and implement its own devicetree bindings model ?

Guenter

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