[PATCH 1/1] docs: admin-guide: fdt and initrd load in EFI stub

[Heinrich Schuchardt <xypron.glpk@gmx.de>]

Describe how a device tree and an initial RAM disk can be passed to the EFI
Boot Stub.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
---
 Documentation/admin-guide/efi-stub.rst | 35 ++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/Documentation/admin-guide/efi-stub.rst b/Documentation/admin-guide/efi-stub.rst
index 833edb0d0bc4..86f50a33884c 100644
--- a/Documentation/admin-guide/efi-stub.rst
+++ b/Documentation/admin-guide/efi-stub.rst
@@ -38,6 +38,34 @@ arch/arm/boot/zImage should be copied to the system partition, and it
 may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
 should be copied but not necessarily renamed.

+Passing an initial RAM disk to the EFI Boot Stub
+------------------------------------------------
+
+The following means sorted by decreasing priority can be used to provide an
+initial RAM disk to the EFI Boot Stub:
+
+* The firmware may provide a UEFI Load File 2 Protocol. The stub will try to
+  load the RAM disk by calling the LoadFile() service of the protocol using
+  a vendor device path with the vendor GUID
+  5568e427-0x68fc-4f3d-ac74-ca555231cc68.
+* Next the EFI stub will try to load the file indicated by the "initrd=" command
+  line parameter.
+* The prior boot stage may pass the location of the initial RAM disk via the
+  "linux,initrd-start" and "linux,initrd-end" properties of the "/chosen" node
+  of the device-tree.
+
+The first two items are inhibited by the "noinitrd" command line parameter.
+
+Passing a device-tree to the EFI Boot Stub
+------------------------------------------
+
+A device-tree can be passed to the EFI Boot Stub in decreasing priority using
+
+* command line option dtb=
+* a UEFI configuration table with GUID b1b621d5-f19c-41a5-830b-d9152c69aae0.
+
+The command line option is only available if CONFIG_EFI_ARMSTUB_DTB_LOADER=y
+and secure boot is disabled.

 Passing kernel parameters from the EFI shell
 --------------------------------------------
@@ -46,6 +74,10 @@ Arguments to the kernel can be passed after bzImage.efi, e.g.::

 	fs0:> bzImage.efi console=ttyS0 root=/dev/sda4

+The "noinitrd" option
+---------------------
+
+The "noinitrd" option stops the EFI stub from loading an initial RAM disk.

 The "initrd=" option
 --------------------
@@ -98,3 +130,6 @@ CONFIGURATION TABLE.

 "dtb=" is processed in the same manner as the "initrd=" option that is
 described above.
+
+This option is only available if CONFIG_EFI_ARMSTUB_DTB_LOADER=y and secure
+boot is disabled.
--
2.28.0