Date:   Mon, 10 Sep 2018 20:08:02 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Olof Johansson <olof@...om.net>
Cc:     Scott Branden <scott.branden@...adcom.com>,
        Grant Likely <grant.likely@...retlab.ca>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Broadcom Kernel Feedback List 
        <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

On 10 September 2018 at 20:01, Olof Johansson <olof@...om.net> wrote:
> On Mon, Sep 10, 2018 at 10:53 AM, Scott Branden
> <scott.branden@...adcom.com> 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@...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?
>
> Revert or Kconfig select, and a Kconfig select means that the option
> is a dead one anyway so we might as well revert.
>

I disagree. Making it default y is fine by me, but please don't remove it.

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

I don't have any fixes but either way is fine.

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