[PATCH v2 1/2] of/fdt: export fdt blob as /sys/firmware/fdt

Grant Likely grant.likely at linaro.org
Wed Nov 12 03:45:10 PST 2014

On Tue, Nov 11, 2014 at 8:34 PM, Ard Biesheuvel
<ard.biesheuvel at linaro.org> wrote:
> On 11 November 2014 21:08, Grant Likely <grant.likely at linaro.org> wrote:
>> On Tue, Nov 11, 2014 at 4:25 PM, Rob Herring <robherring2 at gmail.com> wrote:
>>> On Tue, Nov 11, 2014 at 8:42 AM, Grant Likely <grant.likely at linaro.org> wrote:
>>>> On Mon, 10 Nov 2014 19:47:01 +0100
>>>> , Ard Biesheuvel <ard.biesheuvel at linaro.org>
>>>>  wrote:
>>>>> Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
>>>>> that was passed to the kernel by the bootloader. This allows userland
>>>>> applications such as kexec to access the raw binary. The blob needs to
>>>>> be preserved as early as possible by calling preserve_fdt().
>>>>> The fact that this node does not reside under /sys/firmware/device-tree
>>>>> is deliberate: FDT is also used on arm64 UEFI/ACPI systems to
>>>>> communicate just the UEFI and ACPI entry points, but the FDT is never
>>>>> unflattened and used to configure the system.
>>>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>>>> On further thought, nak. I want tree modifications to be treated as
>>>> bugs. Do a checksum CRC32 or check. It's simpler and it can be extended
>>>> to us an in-blob CRC when the file format gets upreved to include one.
>>> Why? MIPS Octeon is full of them BTW. So the rule is any modifications
>>> or fixups have to be on the unflattened tree?
>> That, or we require the fixups to explicitly clean up the CRC after
>> themselves. That will force them to be visible. Anything using the
>> fdt_*() write functions could potentially take care of it
>> automatically. That would make it immediately discoverable where
>> changes are being made in the tree.
> Doesn't that defeat the purpose of capturing the FDT as it was passed
> by the bootloader? Those fixups will be reapplied by the kexec'ed
> kernel anyway.

We're always going to be in a grey area here. There are some things
that should not be passed over. For example, a framebuffer that is
passed in from firmware will no longer be valid on kexec. Right now on
arch/arm, the exynos platform fixes up the DT because the memory node
is outright /corrupt/. In the MIPS case, they do a bunch of stuff in
the kernel to figure out the hardware as configured by firmware. I
don't agree with that approach, but I would be very surprised if the
full original DT has any validity after the early boot fixups.

Early boot fixups to the .dtb are always going to be oddball cases.


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