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

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Date:	Wed, 6 Jul 2016 12:12:42 +0200
From:	Peter Rosin <peda@...ntia.se>
To:	Rob Herring <robh@...nel.org>
CC:	Guenter Roeck <linux@...ck-us.net>, <devicetree@...r.kernel.org>,
	"Wolfram Sang" <wsa@...-dreams.de>,
	Mark Rutland <mark.rutland@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Kalle Valo <kvalo@...eaurora.org>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	"Jiri Slaby" <jslaby@...e.cz>, <linux-i2c@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dt-bindings: i2c: add bindings for nxp,pca9541

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@...ntia.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...

Cheers,
Peter