linux-kernel - Re: [RFC 0/3] extend kexec_file

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Date:	Wed, 13 Jul 2016 21:57:28 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Mark Rutland <mark.rutland@....com>
Cc:	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	Dave Young <dyoung@...hat.com>, bhe@...hat.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Vivek Goyal <vgoyal@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	bauerman@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 0/3] extend kexec_file_load system call

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

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

> 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.

If you boot a LE kernel using DTB, the same DTB should work
for a kexec boot for a BE kernel.

	Arnd