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[209.132.180.67]) by mx.google.com with ESMTP id b1-v6si1524001pli.54.2018.09.05.02.38.58; Wed, 05 Sep 2018 02:39:13 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@linaro.org header.s=google header.b=JG2cmpZN; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=linaro.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728169AbeIEOGk (ORCPT + 99 others); Wed, 5 Sep 2018 10:06:40 -0400 Received: from mail-it0-f65.google.com ([209.85.214.65]:50597 "EHLO mail-it0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725862AbeIEOGk (ORCPT ); Wed, 5 Sep 2018 10:06:40 -0400 Received: by mail-it0-f65.google.com with SMTP id j81-v6so9023971ite.0 for ; Wed, 05 Sep 2018 02:37:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc; bh=zgFJJAw2nXdZPXJVyfj5bXsAHAS+ETw11QVAoOa7ltw=; b=JG2cmpZNrahhwNT9ifMAxn+4m2lWmBBWY3KYTT4JJYfDQ47c3Y1V+CmeVD9uQNoDov VfYdOj34+Ae2cEUIyJPt6cI4bt8twCXw1O8xNh8UgUnKIlHDHq+U2RyKvsWieMhtLX6/ bwogYysq7o+t1lhhcenQaFht82j4IH7aa4ptg= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:cc; bh=zgFJJAw2nXdZPXJVyfj5bXsAHAS+ETw11QVAoOa7ltw=; b=iwFNPTFnceO+6xAwwGqQ/kfaC+asY/427nGMKErBdqTfuRdJMpq0iotI0BYly3kDVy Tnko1P6lZPJJmqV8ipIRNgUbyO5zlK4aXSdld7+X3sCSOrlbxyRYQZ65+2MNJVzdp7PL G7LRJPEzP0RfpQxzf9LIS5ULiryvjqVcsZJfDYrzcll9VkwKgBbd2/iJFoaJzlJwfWJ9 /+sXcYAn/QmHhCZsz5H2wu4yrxO5ORicL1PjXMh+mdfJD8P/qykWoK2EGKXQcg3iKk4Z tN9MZItcxplvpHMQLTVDb+zJTuRZ7iMhDmM2ZZ5kLr8GWTrCtxOxYa9s1NfIZIY+MaFq JA6Q== X-Gm-Message-State: APzg51A0ruRO7nFV7GhKizshNxUJko1X7/TpgeMULnhDPrxNU7X3ZX8X H+Yc4jl+AohnE9C8YYiARpZiSaJrcSFQsg6+r3vS7w== X-Received: by 2002:a24:d583:: with SMTP id a125-v6mr2917685itg.91.1536140236434; Wed, 05 Sep 2018 02:37:16 -0700 (PDT) MIME-Version: 1.0 Received: by 2002:a6b:1c06:0:0:0:0:0 with HTTP; Wed, 5 Sep 2018 02:37:15 -0700 (PDT) In-Reply-To: References: <1535563287-24803-1-git-send-email-scott.branden@broadcom.com> From: Ard Biesheuvel Date: Wed, 5 Sep 2018 11:37:15 +0200 Message-ID: Subject: Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER To: Grant Likely Cc: Olof Johansson , Scott Branden , Catalin Marinas , Will Deacon , Arnd Bergmann , BCM Kernel Feedback , Linux ARM , Linux Kernel Mailing List , Leif Lindholm , Alexander Graf Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Grant, Thanks for chiming in. On 4 September 2018 at 12:13, 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 wrote: >> >> On 2 September 2018 at 04:54, Olof Johansson wrote: >> > On Thu, Aug 30, 2018 at 9:23 AM, Ard Biesheuvel >> > wrote: >> >> On 30 August 2018 at 17:06, Olof Johansson wrote: >> >>> On Wed, Aug 29, 2018 at 10:54 PM, Ard Biesheuvel >> >>> wrote: >> >>>> On 29 August 2018 at 20:59, Scott Branden 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 >> >>>>>> 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 >> >>>>>> >> >>>>>> 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? > Well, to be honest, I don't have a real-world example at hand, but my concern is about cases where the firmware provided DTB and the override DTB diverge in a way that leaves it up to the EFI stub to reconcile them and/or to reason about which one it should prefer. One example could be OP-TEE support: currently, we put a /firmware/optee node in the DT to inform the OS that OP-TEE is running in the secure world. If we allow a DT to be provided via dtb= that provides such a node, we are blocking all future opportunities in future kernels to do any kind of preparatory OP-TEE related initialization in the EFI stub [while boot services are still available] unless we decide to make it the EFI stub's problem to reason about which version of the DT is the correct one to use. What if the firmware's DT has /firmware/optee/status = disabled and the dtb= one does not? Another trivial example is GRUB: passing dtb= via the command line will break initrds loaded via GRUB, since they are passed via the /chosen node. > 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. > Agreed. So perhaps the conclusion here is that dtb= should be supported fully if no DTB is provided by the firmware in the first place, and only marginally (perhaps with a taint) when the firmware provides one as well. >> >>>> 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. >> > >> >> 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.