Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER

From: Scott Branden
Date: Mon Sep 10 2018 - 13:54:10 EST


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

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 <ard.biesheuvel@xxxxxxxxxx> wrote:
On 2 September 2018 at 04:54, Olof Johansson <olof@xxxxxxxxx> wrote:
On Thu, Aug 30, 2018 at 9:23 AM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
On 30 August 2018 at 17:06, Olof Johansson <olof@xxxxxxxxx> wrote:
On Wed, Aug 29, 2018 at 10:54 PM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
On 29 August 2018 at 20:59, Scott Branden <scott.branden@xxxxxxxxxxxx> wrote:
Hi Olof,

On 18-08-29 11:44 AM, Olof Johansson wrote:

On Wed, Aug 29, 2018 at 10:21 AM, Scott Branden
<scott.branden@xxxxxxxxxxxx> 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 <scott.branden@xxxxxxxxxxxx>
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
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
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).

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
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?
I agree with your analysis but not with your conclusion.

Whether or not the option is def_bool y and/or enabled in defconfig is
a matter of policy. ACPI-only distros such as RHEL are definitely
going to disable this option. But in general, supporting DTBs loaded
from files is a huge pain for the distros, so I expect most of them to
disable it as well.
I support leaving 3d7ee348 in, and making it def_bool y

As for EFI on embedded systems: this will be mostly on U-boot's
bootefi implementation, which definitely does the right thing when it
comes to passing the DTB via a UEFI configuration table (regardless of
whether it makes any modifications to it)

In any case, I won't object to a patch that reenables the EFI stub DTB
loader in defconfig. Whether or not it should be def_bool y is
something we can discuss as well. I have added Leif and Alex to cc,
perhaps they have anything to add.