efi_stub: update documentation on dtb= parameter

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 <[email protected]> Reviewed-by: Alexander Graf <[email protected]> Acked-by: Leif Lindholm <[email protected]> Acked-by: Olof Johansson <[email protected]> Acked-by: Ard Biesheuvel <[email protected]> Signed-off-by: Jonathan Corbet <[email protected]>
teawater · Sep 9, 2018 · 9331e5e · 9331e5e
1 parent 2f4830e
commit 9331e5e
Showing 1 changed file with 14 additions and 3 deletions.
diff --git 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.