On Wed, Jul 13, 2016 at 09:57:28PM +0200, Arnd Bergmann wrote:

On Wednesday, July 13, 2016 6:58:32 PM CEST Mark Rutland wrote:

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  we may want to remove unnecessary devices and even add a dedicated
  storage device for storing a core dump image.

I suspect that bringing up a minimal number of devices is better
controlled by a cmdline option. In general, figuring out what is
necessary and what is not is going to be board specific, so hacking the
FW tables (DTB or ACPI) is not a very portable/reliable approach.

Do we actually add devices in practice? More so than the above that
requires special knowledge of the platform (including things that were
not described in the boot DTB).

In the ACPI case modifying a DTB alone is not sufficient to change the
information regarding devices, as those won't be described in the DTB.
It's not possible to convert ACPI to DTB in general.

A more likely scenario would be replacing ACPI tables with a DTB that
describes the platform in order to use devices that the ACPI tables
don't contain.

To do so, you need special knowledge of the platform, which users are
unlikely to have in practice for ACPI-based platforms.

So, I think that boils down to the same problem, and the same comments
apply.

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- Say, booting BE kernel on ACPI LE kernel
  In this case, there is no useful dtb in the kernel.

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If the platform only has ACPI, then you cannot boot a BE kernel to begin
with. As above one cannot convert ACPI to DTB, so one would need
extensive platform knowledge for this to work.

I think what he meant was to pass a DTB to the kexec kernel in order
to run BE, while the original kernel can only run LE due to ACPI.

I understood that. My point was that to build that DTB, you need to have
knowledge of the platform that you are unlikely to have.

The platform firmware may expect to interact with AML, which you can't
place in DT or run in a BE context.

If you need to run BE kernels on a platform, you would likely get a
platform that could boot a BE kernel from the outset (i.e. one that
provides a DTB), or run that code ina VM under an LE host.

Thanks,
Mark.