On Thu, Nov 26, 2015 at 02:16:16PM +0100, Mason wrote:
> Hello,
>
> In the device tree for my ARM platform, I have several nodes with
> addresses within the SCU block:
>
> scu: scu@20000000 {
> compatible = "arm,cortex-a9-scu";
> reg = <0x20000000 0x100>;
>
> gic: interrupt-controller@20001000 {
> compatible = "arm,cortex-a9-gic";
> reg = <0x20001000 0x1000>, <0x20000100 0x0100>;
>
> twd-timer@20000600 {
> compatible = "arm,cortex-a9-twd-timer";
> reg = <0x20000600 0x10>;
>
> Can I use preprocessor arithmetic to abstract the base address,
> as would be done in C?
>
> #define SCU_BASE 0x20000000
>
> scu: scu@XXX {
> compatible = "arm,cortex-a9-scu";
> reg = <SCU_BASE 0x100>;
>
> gic: interrupt-controller@XXX {
> compatible = "arm,cortex-a9-gic";
> reg = <SCU_BASE+0x1000 0x1000>, <SCU_BASE+0x100 0x0100>;
The pre-processor would only do substitution, not arithmetic. So the '+'
would flow all the way to DTC.
DTC does have some basic integer expression support (see [1]), but it
looks like it can only work at the cell level, so generally it needs to
be avoided (in case #address-cells >1, for example).
So generally I think that such arithmetic should be avoided.
> twd-timer@XXX {
> compatible = "arm,cortex-a9-twd-timer";
> reg = <SCU_BASE+0x600 0x10>;
>
> Are the @XXX important? Can they be removed altogether?
The bit after the @ is called the unit-address.
It's meant to be there (matching the base address of the first reg
entry) to disambiguate nodes and make it simple to figure out where a
node lives in the physical address space.
They should stay.
Thanks,
Mark.
[1] https://git.kernel.org/cgit/linux/kernel/git/jdl/dtc.git/commit/?id=5f0c3b2d6235dec65fff1628a97f45e21680b36d
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