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SubjectRe: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER
On 13 September 2018 at 22:22, Scott Branden <> wrote:
> On 18-09-10 11:08 AM, Ard Biesheuvel wrote:
>> On 10 September 2018 at 20:01, Olof Johansson <> wrote:
>>> On Mon, Sep 10, 2018 at 10:53 AM, Scott Branden
>>> <> wrote:
>>>> Olof/All,
>>>> On 18-09-04 03:13 AM, Grant Likely wrote:
>>>>> Hey folks. More comments below, but the short answer is I really don't
>>>>> see what the problem is. Distros cannot easily support platforms that
>>>>> require a dtb= parameter, and so they probably won't. They may or may
>>>>> not disable 'dtb=', depending on whether they see it as valuable for
>>>>> debug.
>>>>> Vertically integrated platforms are a different beast. We may strongly
>>>>> recommend firmware provides the dtb for all the mentioned good
>>>>> reasons, but they still get to decide their deployment methodology,
>>>>> and it is not burdensome for the kernel to keep the dtb= feature that
>>>>> they are using.
>>>>> On Tue, Sep 4, 2018 at 7:24 AM Ard Biesheuvel
>>>>> <>
>>>>> wrote:
>>>>>> On 2 September 2018 at 04:54, Olof Johansson <> wrote:
>>>>>>> On Thu, Aug 30, 2018 at 9:23 AM, Ard Biesheuvel
>>>>>>> <> wrote:
>>>>>>>> On 30 August 2018 at 17:06, Olof Johansson <> wrote:
>>>>>>>>> On Wed, Aug 29, 2018 at 10:54 PM, Ard Biesheuvel
>>>>>>>>> <> wrote:
>>>>>>>>>> On 29 August 2018 at 20:59, Scott Branden
>>>>>>>>>> <> wrote:
>>>>>>>>>>> Hi Olof,
>>>>>>>>>>> On 18-08-29 11:44 AM, Olof Johansson wrote:
>>>>>>>>>>>> Hi,
>>>>>>>>>>>> On Wed, Aug 29, 2018 at 10:21 AM, Scott Branden
>>>>>>>>>>>> <> wrote:
>>>>>>>>>>>>> Enable EFI_ARMSTUB_DTB_LOADER to add support for the dtb=
>>>>>>>>>>>>> command
>>>>>>>>>>>>> line
>>>>>>>>>>>>> parameter to function with efi loader.
>>>>>>>>>>>>> Required to boot on existing bootloaders that do not support
>>>>>>>>>>>>> devicetree
>>>>>>>>>>>>> provided by the platform or by the bootloader.
>>>>>>>>>>>>> Fixes: 3d7ee348aa41 ("efi/libstub/arm: Add opt-in Kconfig
>>>>>>>>>>>>> option
>>>>>>>>>>>>> for the
>>>>>>>>>>>>> DTB loader")
>>>>>>>>>>>>> Signed-off-by: Scott Branden <>
>>>>>>>>>>>> Why did Ard create an option for this if it's just going be
>>>>>>>>>>>> turned
>>>>>>>>>>>> on
>>>>>>>>>>>> in default configs? Doesn't make sense to me.
>>>>>>>>>>>> It would help to know what firmware still is crippled and how
>>>>>>>>>>>> common
>>>>>>>>>>>> it is, since it's been a few years that this has been a
>>>>>>>>>>>> requirement
>>>>>>>>>>>> by
>>>>>>>>>>>> now.
>>>>>>>>>>> Broadcom NS2 and Stingray in current development and production
>>>>>>>>>>> need
>>>>>>>>>>> this
>>>>>>>>>>> option in the kernel enabled in order to boot.
>>>>>>>>>> And these production systems run mainline kernels in a defconfig
>>>>>>>>>> configuration?
>>>>>>>>>> The simply reality is that the DTB loader has been deprecated for
>>>>>>>>>> a
>>>>>>>>>> good reason: it was only ever intended as a development hack
>>>>>>>>>> anyway,
>>>>>>>>>> and if we need to treat the EFI stub provided DTB as a first class
>>>>>>>>>> citizen, there are things we need to fix to make things works as
>>>>>>>>>> expected. For instance, GRUB will put a property in the /chosen
>>>>>>>>>> node
>>>>>>>>>> for the initramfs which will get dropped if you boot with dtb=.
>>>>>>>>>> Don't be surprised if some future enhancements of the EFI stub
>>>>>>>>>> code
>>>>>>>>>> depend on !EFI_ARMSTUB_DTB_LOADER.
>>>>> That's an odd statement to make. The DTB loader code is well contained
>>>>> and with defined semantics... True, the semantics are "I DON'T BELIEVE
>>>>> FIRMWARE", but it is still well defined. What scenario are you
>>>>> envisioning where EFI_ARMSTUB_DTB_LOADER would be explicitly excluded?
>>>>> Conversely, the dtb= argument is an invaluable debug tool during
>>>>> development. As Olof has already said, there are a lot of embedded
>>>>> deployments where there is no desire for grub or any other
>>>>> intermediary loader.
>>>>>>>>>> On UEFI systems, DTBs [or ACPI
>>>>>>>>>> tables] are used by the firmware to describe itself and the
>>>>>>>>>> underlying
>>>>>>>>>> platform to the OS, and the practice of booting with DTB file
>>>>>>>>>> images
>>>>>>>>>> (taken from the kernel build as well) conflicts with that view.
>>>>>>>>>> Note
>>>>>>>>>> that GRUB still permits you to load DTBs from files (and supports
>>>>>>>>>> more
>>>>>>>>>> sources than just the file system the kernel Image was loaded
>>>>>>>>>> from).
>>>>>>>>> Ard,
>>>>>>>>> Maybe a WARN() splat would be more useful as a phasing-out method
>>>>>>>>> than
>>>>>>>>> removing functionality for them that needs to be reinstated through
>>>>>>>>> changing the config?
>>>>>>>> We don't have any of that in the stub, and inventing new ways to
>>>>>>>> pass
>>>>>>>> such information between the stub and the kernel proper seems like a
>>>>>>>> cart-before-horse kind of thing to me. The EFI stub diagnostic
>>>>>>>> messages you get on the serial console are not recorded in the
>>>>>>>> kernel
>>>>>>>> log buffer, so they only appear if you actually look at the serial
>>>>>>>> output.
>>>>> As an aside, they probably should be recorded. That is probably a
>>>>> question for the UEFI USWG. Grub and the ARMSTUB could probably bodge
>>>>> something together, but that would be non-standard.
>>>>>>> Ah yeah. I suppose you could do it in the kernel later if you detect
>>>>>>> you've booted through EFI with dtb= on the command line though.
>>>>>>>>> Once the stub and the boot method is there, it's hard to undo as we
>>>>>>>>> can see here. Being loud and warn might be more useful, and set a
>>>>>>>>> timeline for hard removal (12 months?).
>>>>>>>> The dtb= handling is still there, it is just not enabled by default.
>>>>>>>> We can keep it around if people are still using it. But as I pointed
>>>>>>>> out, we may decide to make new functionality available only if it is
>>>>>>>> disabled, and at that point, we'll have to choose between one or the
>>>>>>>> other in defconfig, which is annoying.
>>>>>>>>> Scott; an alternative for you is to do a boot wrapper that bundles
>>>>>>>>> a
>>>>>>>>> DT and kernel, and boot that instead of the kernel image (outside
>>>>>>>>> of
>>>>>>>>> the kernel tree). Some 32-bit platforms from Marvell use that. That
>>>>>>>>> way the kernel will just see it as a normally passed in DT.
>>>>>>>> Or use GRUB. It comes wired up in all the distros, and let's you
>>>>>>>> load
>>>>>>>> a DT binary from anywhere you can imagine, as opposed to the EFI
>>>>>>>> stub
>>>>>>>> which can only load it if it happens to reside in the same file
>>>>>>>> system
>>>>>>>> (or even directory - I can't remember) as the kernel image. Note
>>>>>>>> that
>>>>>>>> the same reservations apply to doing that - the firmware is no
>>>>>>>> longer
>>>>>>>> able to describe itself to the OS via the DT, which is really the
>>>>>>>> only
>>>>>>>> conduit it has available on an arm64 system..
>>>>>>> So, I've looked at the history here a bit, and dtb= support was
>>>>>>> introduced in 2014. Nowhere does it say that it isn't a recommended
>>>>>>> way of booting.
>>>>>>> There are some firmware stacks today that modify and provide a
>>>>>>> runtime-updated devicetree to the kernel, but there are also a bunch
>>>>>>> who don't. Most "real" products will want a firmware that knows how
>>>>>>> to
>>>>>>> pass in things such as firmware environment variables, or MAC
>>>>>>> addresses, etc, to the kernel, but not all of them need it.
>>>>>>> In particular, in a world where you want EFI to be used on embedded
>>>>>>> platforms, requiring another bootloader step such as GRUB to be able
>>>>>>> to reasonably boot said platforms seems like a significant and
>>>>>>> unfortunate new limitation. Documentation/efi-stub.txt has absolutely
>>>>>>> no indication that it is a second-class option that isn't expected to
>>>>>>> be available everywhere. It doesn't really matter what _your_
>>>>>>> intention was around it, if those who use it never found out and now
>>>>>>> rely on it.
>>>>>>> Unfortunately the way forward here is to revert 3d7ee348aa4127a.
>>>> What's the path forward? Revert, defconfig change (this patch), or
>>>> Kconfig
>>>> default addition?
>>> Revert or Kconfig select, and a Kconfig select means that the option
>>> is a dead one anyway so we might as well revert.
>> I disagree. Making it default y is fine by me, but please don't remove it.
>>> Ard, do you have other fixes lined up or should we take the patch
>>> through arm-soc?
>> I don't have any fixes but either way is fine.
> I submitted the version of the patch Ard requested here for somebody to pick
> up.

Thanks Scott. I will pick it up as a EFI fix.

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