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

Grant Likely grant.likely at linaro.org
Wed Nov 12 04:10:47 PST 2014

On Wed, Nov 12, 2014 at 12:08 PM, Ard Biesheuvel
<ard.biesheuvel at linaro.org> wrote:
> On 12 November 2014 13:01, Grant Likely <grant.likely at linaro.org> wrote:
>> On Wed, Nov 12, 2014 at 11:51 AM, Ard Biesheuvel
>> <ard.biesheuvel at linaro.org> wrote:
>>> On 12 November 2014 12:45, Grant Likely <grant.likely at linaro.org> wrote:
>>>> 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.
>>> Well, yes, but those fixups will presumably be done on every boot,
>>> including the kexec ones.
>>>> Early boot fixups to the .dtb are always going to be oddball cases.
>>> OK, so generally speaking, whether the original DTB is more suitable
>>> to be presented through /sys/firmware/fdt than the 'current' FDT
>>> (i.e., whatever is in memory at the time the sysfs node is accessed)
>>> is not obvious at all. So why not just do the latter?
>> We're now talking theoreticals. In the case you're working on
>> (ACPI+FDT), the FDT is generated by Grub or the EFI stub as an
>> intermediary format, and there isn't currently any code in your boot
>> path that modifies the FDT. You're arguing for a future situation
>> where the kernel makes a fixup that is not wanted in the second
>> kernel. I'm arguing that 1) I want to /know/ when that happens because
>> there are very few situations where it is a valid thing to do, and 2)
>> the likelyhood of that happening in the FDT+ACPI case is somewhere
>> between slim and none because you're working with an intermediary FDT,
>> not a real hardware description.
> But do you still want the warning? Or only if you access the sysfs
> node? Or just a pr_warn() perhaps rather than a full blown WARN() ?
> MIPS will start seeing the warning as well even if they don't care
> about what is in /sys/firmware/fdt

pr_warn() and then don't export the file in /sys/firmware/fdt. I'm
okay with the check being done once at the end of initcalls.


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