Messages in this thread | | | Subject | Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER | From | Scott Branden <> | Date | Mon, 10 Sep 2018 10:53:40 -0700 |
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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 <ard.biesheuvel@linaro.org> wrote: >> On 2 September 2018 at 04:54, Olof Johansson <olof@lixom.net> wrote: >>> On Thu, Aug 30, 2018 at 9:23 AM, Ard Biesheuvel >>> <ard.biesheuvel@linaro.org> wrote: >>>> On 30 August 2018 at 17:06, Olof Johansson <olof@lixom.net> wrote: >>>>> On Wed, Aug 29, 2018 at 10:54 PM, Ard Biesheuvel >>>>> <ard.biesheuvel@linaro.org> wrote: >>>>>> On 29 August 2018 at 20:59, Scott Branden <scott.branden@broadcom.com> 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 >>>>>>>> <scott.branden@broadcom.com> 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@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. > 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? >> 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 > > g. >> 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. Thanks, Scott
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