linux-kernel - Re: [PATCH RESEND] efi_stub: update documentation on dtb= parameter

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Message-ID: <CAKv+Gu_k5Yr-aRhE6atrtGQOVpw60XfevVdUb8BvH1G3mfoU+Q@mail.gmail.com>
Date:   Fri, 7 Sep 2018 12:43:25 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Grant Likely <grant.likely@....com>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Olof Johansson <olof@...om.net>,
        Scott Branden <scott.branden@...adcom.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Leif Lindholm <leif.lindholm@...aro.org>,
        Alexander Graf <agraf@...e.de>, nd <nd@....com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH RESEND] efi_stub: update documentation on dtb= parameter

On 6 September 2018 at 20:29, Grant Likely <grant.likely@....com> wrote:
> The dtb= parameter is no longer the primary mechanism for providing a
> devicetree to the kernel. Now either firmware or the boot selector (ex.
> Grub) should provide the devicetree and dtb= should only be used for
> debug or when using firmware that doesn't understand DT.
> Update the EFI stub documentation to reflect the current usage.
>
> Signed-off-by: Grant Likely <grant.likely@....com>
> Reviewed-by: Alexander Graf <agraf@...e.de>
> Acked-by: Leif Lindholm <leif.lindholm@...aro.org>
> Acked-by: Olof Johansson <olof@...om.net>
> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: Jonathan Corbet <corbet@....net>

Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>


> ---
>
> Resending because original post included Arm corporate disclaimer.
>
>  Documentation/efi-stub.txt | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/efi-stub.txt b/Documentation/efi-stub.txt
> index 41df801f9a50..833edb0d0bc4 100644
> --- a/Documentation/efi-stub.txt
> +++ b/Documentation/efi-stub.txt
> @@ -83,7 +83,18 @@ is passed to bzImage.efi.
>  The "dtb=" option
>  -----------------
>
> -For the ARM and arm64 architectures, we also need to be able to provide a
> -device tree to the kernel. This is done with the "dtb=" command line option,
> -and is processed in the same manner as the "initrd=" option that is
> +For the ARM and arm64 architectures, a device tree must be provided to
> +the kernel. Normally firmware shall supply the device tree via the
> +EFI CONFIGURATION TABLE. However, the "dtb=" command line option can
> +be used to override the firmware supplied device tree, or to supply
> +one when firmware is unable to.
> +
> +Please note: Firmware adds runtime configuration information to the
> +device tree before booting the kernel. If dtb= is used to override
> +the device tree, then any runtime data provided by firmware will be
> +lost. The dtb= option should only be used either as a debug tool, or
> +as a last resort when a device tree is not provided in the EFI
> +CONFIGURATION TABLE.
> +
> +"dtb=" is processed in the same manner as the "initrd=" option that is
>  described above.
> --
> 2.11.0
>