Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER

[Olof Johansson]

On Mon, Sep 10, 2018 at 10:53 AM, Scott Branden
<scott.branden@xxxxxxxxxxxx> 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 <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:
>>>>>>>>>
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> 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
>>>>>>>>> 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.

Ard, do you have other fixes lined up or should we take the patch
through arm-soc?


Thanks,


-Olof