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Subject: Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Olof Johansson <olof@lixom.net>
Cc:     Grant Likely <grant.likely@secretlab.ca>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Broadcom Kernel Feedback List 
        <bcm-kernel-feedback-list@broadcom.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Leif Lindholm <leif.lindholm@linaro.org>,
        Alexander Graf <agraf@suse.de>
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From:   Scott Branden <scott.branden@broadcom.com>
Message-ID: <b711f74c-363f-4f41-bb1e-a9550147758a@broadcom.com>
Date:   Thu, 13 Sep 2018 13:22:16 -0700
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On 18-09-10 11:08 AM, Ard Biesheuvel wrote:
> On 10 September 2018 at 20:01, Olof Johansson <olof@lixom.net> wrote:
>> On Mon, Sep 10, 2018 at 10:53 AM, Scott Branden
>> <scott.branden@broadcom.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@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?
>> 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.
I submitted the version of the patch Ard requested here for somebody to 
pick up.
https://lore.kernel.org/patchwork/patch/984521/