On Thu, Aug 30, 2018 at 9:23 AM, Ard Biesheuvel <[email protected]> wrote: > On 30 August 2018 at 17:06, Olof Johansson <[email protected]> wrote: >> On Wed, Aug 29, 2018 at 10:54 PM, Ard Biesheuvel >> <[email protected]> wrote: >>> On 29 August 2018 at 20:59, Scott Branden <[email protected]> >>> 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 >>>>> <[email protected]> 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 <[email protected]> >>>>> >>>>> 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. > >> 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. -Olof
