Date:   Mon, 10 Sep 2018 10:53:40 -0700
From:   Scott Branden <scott.branden@...adcom.com>
To:     Grant Likely <grant.likely@...retlab.ca>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Olof Johansson <olof@...om.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        bcm-kernel-feedback-list@...adcom.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Leif Lindholm <leif.lindholm@...aro.org>,
        Alexander Graf <agraf@...e.de>
Subject: Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER

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