[PATCH v6 09/25] arm: xenguest_arm64: Add a fake devicetree file

[Oleksandr Andrushchenko]

Hi, Simon!

On 02.12.21 19:57, Simon Glass wrote:
> Hi Oleksandr,
>
> On Thu, 2 Dec 2021 at 10:40, Oleksandr Andrushchenko
> <Oleksandr_Andrushchenko at epam.com> wrote:
>> Hi, Simon!
>>
>> Sorry for being late to the party
>>
>> On 02.12.21 17:59, Simon Glass wrote:
>>> Add an empty file to prevent build errors when building with
>>> CONFIG_OF_SEPARATE enabled.
>>>
>>> The build instructions in U-Boot do not provide enough detail to build a
>>> useful devicetree, unfortunately.
>> Xen guest doesn't use any built-in device trees as the guest's device tree is provided
>> by the Xen hypervisor itself and is generated at the virtual machine creation time: it is
>> populated with memory size, number of CPUs etc. based on [1].
>> So, even if we provide some device tree here it must not be used by U-boot at
>> the end of the day. Thus, it might be a reasonable solution to provide an empty device
>> tree as you do, but put a comment that it is not used.
> OK we can go with an empty one if we have to, but where are the
> instructions to create the DT that is used?
You don't need to create the device tree yourself, but instead it is
provided by Xen and generated at run-time while creating a
virtual machine. So, it is up to Xen to provide one.
There are cases [1] when you may want providing a so called
partial device tree to better tune what a virtual machine gets.
But again, it is used by Xen toolstack outside of the virtual machine
and serves as a sort of overlay to the generated device tree.
So, we can provide some device tree to be embedded in U-boot,
but it will have no practical meaning and will make more harm than good
>
> I'm not even sure how to run U-Boot with Xen? The in-tree instructions
> don't help...
This is just a virtual machine from Xen POV, so U-boot is nothing
different here from Linux kernel or anything else.
Thus no specific instructions are needed nor provided

Thank you,
Oleksandr
>
> Regards,
> Simon
[1] https://xenbits.xen.org/docs/unstable/misc/arm/passthrough.txt