linux-kernel - RE: [PATCHv2 1/3] dt/bindings: Add the DT binding documentation for endianness

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Date:	Wed, 30 Apr 2014 05:08:46 +0000
From:	"Li.Xiubo@...escale.com" <Li.Xiubo@...escale.com>
To:	Mark Rutland <mark.rutland@....com>
CC:	"broonie@...nel.org" <broonie@...nel.org>,
	"rob@...dley.net" <rob@...dley.net>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCHv2 1/3] dt/bindings: Add the DT binding documentation for
 endianness


> > diff --git a/Documentation/devicetree/bindings/endianness/endianness.txt
> b/Documentation/devicetree/bindings/endianness/endianness.txt
> > new file mode 100644
> > index 0000000..64f1d5e
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/endianness/endianness.txt
> > @@ -0,0 +1,55 @@
> > +Device-Tree binding for device endianness
> > +
> > +The endianness mode of CPU & Device scenarios:
> > +  Index    CPU      Device
> > +  ------------------------
> > +  1        LE         LE
> > +  2        LE         BE
> > +  3        BE         BE
> > +  4        BE         LE
> > +
> > +For one device driver, which will run in different scenarios above
> > +on different SoCs using the devicetree, we need one way to simplify
> > +this.
> > +
> > +Required properties:
> > +- [prefix]-endian: this is one string property and must be one
> > +  of 'be', 'le' and 'native' if it is present.
> 
> What exactly is the prefix?
> 
> This file name and file heading implies this is a common endianness
> binding, which it is not. There are many existing bindings that have
> mechanisms for describing the endianness of components, and they have
> settled on a different pattern.
> 
> We already have many bindings with {big,little}-endian{,-*} boolean
> properties. It would be better to take that common case and document
> that as the standard way of doing things rather than inventing a
> completely different mechanism.
> 

Well, yes, I'll follow your advice.


> > +The endianness mode:
> > +  'le' : device is in little endian mode,
> > +  'be' : device is in big endian mode,
> > +  'native' : device is in the same endian mode with the CPU.
> 
> What exactly do you mean by native? A device which always matches the
> endianness of the CPU even if it changes? How common is that?
> 

It's originally based the regmap, and the 'native' is make no sense here,
I'll reconstruct this.


> > +
> > +Examples:
> > +Scenario 1 : CPU in LE mode & device in LE mode.
> > +dev: dev@...31000 {
> > +	      compatible = "name";
> > +	      reg = <0x40031000 0x1000>;
> > +	      ...
> > +	      [prefix]-endian = 'native';
> > +};
> 
> If the device is LE, then surely we should describe the device as LE.
> The kernel knows what endianness it is, might choose to change the CPU
> endianness, but regardless can do the right thing. Telling it a device
> is native is useless unless the device changes endianness with the CPU.
> 

Yes, you are right.

> > +
> > +Scenario 2 : CPU in LE mode & device in BE mode.
> > +dev: dev@...31000 {
> > +	      compatible = "name";
> > +	      reg = <0x40031000 0x1000>;
> > +	      ...
> > +	      [prefix]-endian = 'be';
> > +};
> > +
> > +Scenario 3 : CPU in BE mode & device in BE mode.
> > +dev: dev@...31000 {
> > +	      compatible = "name";
> > +	      reg = <0x40031000 0x1000>;
> > +	      ...
> > +	      [prefix]-endian = 'native';
> > +};
> 
> Likewise.
> 
> Thanks,
> Mark.
> 

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