On Thu, Aug 30, 2018 at 9:23 AM, Ard Biesheuvel
[off-list ref] wrote:

On 30 August 2018 at 17:06, Olof Johansson [off-list ref] wrote:

quoted

On Wed, Aug 29, 2018 at 10:54 PM, Ard Biesheuvel
[off-list ref] wrote:

quoted

On 29 August 2018 at 20:59, Scott Branden [off-list ref] wrote:

quoted

Hi Olof,


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

quoted

Hi,

On Wed, Aug 29, 2018 at 10:21 AM, Scott Branden
[off-list ref] wrote:

quoted

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@broadcom.com>

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. 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.

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.

quoted

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.

quoted

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.


-Olof