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

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Date:	Mon, 27 Jun 2016 06:17:36 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Peter Rosin <peda@...ntia.se>, devicetree@...r.kernel.org
Cc:	Wolfram Sang <wsa@...-dreams.de>, Rob Herring <robh+dt@...nel.org>,
	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 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.

> 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
>