[U-Boot] Accessing nested device tree nodes via phandle

[Simon Glass]

Hi Vlad,

On 20 December 2016 at 08:53, Vlad Zakharov
<Vladislav.Zakharov at synopsys.com> wrote:
> Hi everybody,
>
> I am working on reorganization of device trees for ARC boards in U-Boot in order to create a common device tree blob for
> both Linux and U-Boot.
>
> And I've struggled with a kind of problem and need some help.
>
> On AXS10x motherboard clocks for various peripherals are managed by one hardware block, so clock nodes in Linux device
> tree are grouped into one "clocks" node like this:
> ---------------------------------->8------------------------------------
> clocks {
>     i2cclk: i2cclk {
>         compatible = "fixed-clock";
>         clock-frequency = <50000000>;
>         #clock-cells = <0>;
>     };
>  ...
>  ...
>  ...
>     uartclk: uartclk {
>         compatible = "fixed-clock";
>         clock-frequency = <33333333>;
>         #clock-cells = <0>;
>     };
> };
> ---------------------------------->8------------------------------------
> And then some of them a referenced via phandle from other nodes, e.g. &uartclk is referenced from uart node.
>
> When I try to create such hierarchy in U-Boot I find that uart driver is not able to get clock via phandle and
> corresponding uart doesn't work.
>
> As I understand that is because children clocks nodes, i.e. "uartclk" in my case, never got bound as it's parent node
> "clocks" never got bound too.
> So when I add "compatible" and "u-boot,dm-pre-reloc" properties to "clocks" parent node everything works fine, e.g.:
> ---------------------------------->8------------------------------------
> clocks {
>     compatible = "simple-bus";
>     u-boot,dm-pre-reloc;
>     uartclk: uartclk {
>         compatible = "fixed-clock";
>
>        clock-frequency = <33333333>;
>         #clock-cells = <0>;
>     };
>  ...
>  ...
>  ...
> };
> ---------------------------------->8------------------------------------
>
> Please correct me if I wrongly interpret such behavior.
>
> I wonder if it the only approach to implement Linux-like hierarchy for ARC? Or I am mistaken and in fact can use pure
> Linux device tree without adding properties, described above?

Yes you have it right. I'm not sure what linux is doing here - does it
look for a node called 'clocks'? To my mind, U-Boot is the more
correct.

Regards,
Simon