Re: [Xen-users] Modifying fdt in the bootscript v.s. in the dts

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Sat, 2015-02-14 at 14:11 -0500, Jintack Lim wrote:
> Hi all,
> 
> This question could be more about an u-boot than Xen,
> however it's based on xen wiki, so it might be a valid question to xen
> community.
> My question is,
> when do we modify dts directly and when do we modify device tree in
> the boot script?
> 
> For example,
> in cubietruck case, a boot script adds address-cells and size-cells to
> chosen like this.
> http://wiki.xen.org/wiki/Xen_ARM_with_Virtualization_Extensions/Allwinner
> fdt set /chosen \#address-cells <1>
> fdt set /chosen \#size-cells <1>
> 
> However, in mustang case, the wiki recommends to add address-cells,
> size cells to the device tree (dts file).
> http://wiki.xen.org/wiki/Xen_ARM_with_Virtualization_Extensions/APMXGeneMustang
> 
> I'm not sure this is just a taste, or is there any underlying principle?

Mostly taste, but IMHO the instructions should be written to assume that
the FDT is immutable, i.e. burned into firmware and therefore do any
necessary updates in the scripts.

In reality the FDT is not often actual burnt into the firmware, but from
a deployment/distro packaging PoV it is also much more convenient if the
Xen boot procedure can start from the same binary FDT as is shipped in
the distro or with the kernel etc, so treating it as immutable is
helpful overall.

Ian.


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