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From:   Grant Likely <grant.likely@arm.com>
To:     linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
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        catalin.marinas@arm.com, will.deacon@arm.com, arnd@arndb.de,
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        Grant Likely <grant.likely@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jonathan Corbet <corbet@lwn.net>
Subject: [PATCH] efi_stub: update documentation on dtb= parameter
Date:   Wed,  5 Sep 2018 20:07:50 +0100
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The dtb=3D 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=3D 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>
---
 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=3D" 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=3D" command line opt=
ion,
-and is processed in the same manner as the "initrd=3D" 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=3D" 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=3D is used to override
+the device tree, then any runtime data provided by firmware will be
+lost. The dtb=3D 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=3D" is processed in the same manner as the "initrd=3D" option that is
 described above.
--
2.11.0

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