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Date:   Wed, 5 Sep 2018 20:30:28 +0100
From:   Leif Lindholm <leif.lindholm@linaro.org>
To:     Grant Likely <grant.likely@arm.com>
Cc:     linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, olof@lixom.net,
        scott.branden@broadcom.com, catalin.marinas@arm.com,
        will.deacon@arm.com, arnd@arndb.de, agraf@suse.de,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH] efi_stub: update documentation on dtb= parameter
Message-ID: <20180905193028.4vhqtlwwgvqq6v26@bivouac.eciton.net>
References: <20180905190750.4566-1-grant.likely@arm.com>
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On Wed, Sep 05, 2018 at 08:07:50PM +0100, Grant Likely 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@arm.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: Jonathan Corbet <corbet@lwn.net>

Acked-by: Leif Lindholm <leif.lindholm@linaro.org>

> ---
>  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
> 
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