[PATCH 00/16] fdt: Make OF_BOARD a boolean option

[Philippe Mathieu-Daudé]

Hi Simon,

On 10/13/21 03:29, Bin Meng wrote:
> On Wed, Oct 13, 2021 at 9:01 AM Simon Glass <sjg at chromium.org> wrote:
>>
>> With Ilias' efforts we have dropped OF_PRIOR_STAGE and OF_HOSTFILE so
>> there are only three ways to obtain a devicetree:
>>
>>    - OF_SEPARATE - the normal way, where the devicetree is built and
>>       appended to U-Boot
>>    - OF_EMBED - for development purposes, the devicetree is embedded in
>>       the ELF file (also used for EFI)
>>    - OF_BOARD - the board figures it out on its own
>>
>> The last one is currently set up so that no devicetree is needed at all
>> in the U-Boot tree. Most boards do provide one, but some don't. Some
>> don't even provide instructions on how to boot on the board.
>>
>> The problems with this approach are documented at [1].
>>
>> In practice, OF_BOARD is not really distinct from OF_SEPARATE. Any board
>> can obtain its devicetree at runtime, even it is has a devicetree built
>> in U-Boot. This is because U-Boot may be a second-stage bootloader and its
>> caller may have a better idea about the hardware available in the machine.
>> This is the case with a few QEMU boards, for example.
>>
>> So it makes no sense to have OF_BOARD as a 'choice'. It should be an
>> option, available with either OF_SEPARATE or OF_EMBED.
>>
>> This series makes this change, adding various missing devicetree files
>> (and placeholders) to make the build work.
> 
> Adding device trees that are never used sounds like a hack to me.
> 
> For QEMU, device tree is dynamically generated on the fly based on
> command line parameters, and the device tree you put in this series
> has various hardcoded <phandle> values which normally do not show up
> in hand-written dts files.

Besides, QEMU generates these dtb at runtime on purpose: it gives
emulated machines the freedom to evolve by adding new devices,
mapping/wiring peripherals differently.

By adding static dtb this gives QEMU users false expectations the
machine hardware is stable, or force QEMU to have this interface
become a stable API.

>From QEMU perspective this seems counter-productive.

Digging a bit I see this has already been discussed on qemu-devel@
mailing list recently:

https://lore.kernel.org/qemu-devel/CAFEAcA_QNcAHtdxUPLpmyzMYgb9YzhcE0-zyh=N8rqm4vOcGZA@mail.gmail.com/

> I am not sure I understand the whole point of this.
> 
>>
>> It also provides a few qemu clean-ups discovered along the way.
>>
>> This series is based on Ilias' two series for OF_HOSTFILE and
>> OF_PRIOR_STAGE removal.
>>
>> It is available at u-boot-dm/ofb-working
>>
>> [1] https://patchwork.ozlabs.org/project/uboot/patch/20210919215111.3830278-3-sjg@chromium.org/
>>
> 
> Regards,
> Bin
>