Re: [PATCH 05/11] ARM: shmobile: APE6EVM LAN9220 support

[Arnd Bergmann <arnd@arndb.de>]

On Thursday 30 May 2013, Guennadi Liakhovetski wrote:
> Ok, I didn't write the original implementation of that bus and, therefore, 
> didn't decide how many address cells it shall use. My task was to fix it. 
> And yes, I agree, that there are 2 ways to fix the memory mapping: either 
> define ranges to map from 64-bits to 32-bits, or to change all devices on 
> the bus (currently there's only 1) to use 64-bit "reg" property and use 
> "ranges" with no arguments. Since that bus is clearly within a 32-bit 
> area, I would prefer the former solution not to have to drag 64 bits 
> around an make device nodes simpler. Can we keep this approach or are you 
> strongly against it?

I believe we should attempt to have the DT representation as close as
possible to what the hardware actually does. If this is a AHB bus
or something else that is known to be always 32 bit, I would use
a ranges property and #address-cells=<1>, but if this is a 64-bit
wide AXIv4 instance I would use #address-cells=<2> and probably
omit the ranges.

There is also the case where a device is attached to an external bus
that has 0-based addressing. In that case I would put the actual
translation into the ranges property, with all addresses that
are translated by the bus controller, such as

      lbsc {
               compatible = "simple-bus";
               #address-cells = <1>;
               #size-cells = <1>;
               ranges = <0 0x08000000 0 0x1000000>; /* 16 MB @ 128 MB */

               ethernet@0 {
			compatible = "smsc,lan9118", "smsc,lan9115";
			reg = <0 0x1000>;
			...

	Arnd