cmd: pxe: add alias devicetree-overlay for fdtoverlays

This adds keyword devicetree-overlay as an alias for fdtoverlays in
extlinux (sysboot) and pxe to better follow the Boot Loader Specification
[1], improves documentation around them by adding an example for both
fdtoverlays and devicetree-overlay and the environment variable required
for this feature. The link for the spec is updated to the current one.

[1] https://systemd.io/BOOT_LOADER_SPECIFICATION/

Signed-off-by: Edoardo Tomelleri <e.tomell@gmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>

1 files changed, 29 insertions, 3 deletions

diff --git a/doc/develop/distro.rst b/doc/develop/distro.rst
index 3ee3dac6a2..bc72aa951e 100644
--- a/doc/develop/distro.rst
+++ b/doc/develop/distro.rst
@@ -68,10 +68,10 @@ Boot Configuration Files
 
 The standard format for boot configuration files is that of extlinux.conf, as
 handled by U-Boot's "syslinux" (disk) or "pxe boot" (network). This is roughly
-as specified at BootLoaderSpec_:
+as specified at `Boot Loader Specification`_:
 
 
-... with the exceptions that the BootLoaderSpec document:
+... with the exceptions that the Boot Loader Specification document:
 
 * Prescribes a separate configuration per boot menu option, whereas U-Boot
   lumps all options into a single extlinux.conf file. Hence, U-Boot searches
@@ -81,6 +81,8 @@ as specified at BootLoaderSpec_:
 * Does not document the fdtdir option, which automatically selects the DTB to
   pass to the kernel.
 
+See also doc/README.pxe under 'pxe file format'.
+
 One example extlinux.conf generated by the Fedora installer is::
 
     # extlinux.conf generated by anaconda
@@ -115,6 +117,25 @@ One example extlinux.conf generated by the Fedora installer is::
         fdtdir /boot/dtb-3.16.0-0.rc6.git1.1.fc22.armv7hl+lpae
 
 
+One example of hand-crafted extlinux.conf::
+
+   menu title Select kernel
+   timeout 100
+
+   label Arch with uart devicetree overlay
+       kernel /arch/Image.gz
+       initrd /arch/initramfs-linux.img
+       fdt /dtbs/arch/board.dtb
+       fdtoverlays /dtbs/arch/overlay/uart0-gpio0-1.dtbo
+       append console=ttyS0,115200 console=tty1 rw root=UUID=fc0d0284-ca84-4194-bf8a-4b9da8d66908
+
+   label Arch with uart devicetree overlay but with Boot Loader Specification keys
+       kernel /arch/Image.gz
+       initrd /arch/initramfs-linux.img
+       devicetree /dtbs/arch/board.dtb
+       devicetree-overlay /dtbs/arch/overlay/uart0-gpio0-1.dtbo
+       append console=ttyS0,115200 console=tty1 rw root=UUID=fc0d0284-ca84-4194-bf8a-4b9da8d66908
+
 Another hand-crafted network boot configuration file is::
 
     TIMEOUT 100
@@ -214,6 +235,11 @@ fdt_addr_r:
 
   A size of 1MB for the FDT/DTB seems reasonable.
 
+fdtoverlay_addr_r:
+  Mandatory. The location in RAM where DTB overlays will be temporarily
+  stored and then applied in the load order to the fdt blob stored at the
+  address indicated in the fdt_addr_r environment variable.
+
 fdtfile:
   Mandatory. the name of the DTB file for the specific board for instance
   the espressobin v5 board the value is "marvell/armada-3720-espressobin.dtb"
@@ -408,7 +434,7 @@ way in future u-boot versions.  In particular the <device type>_boot
 variables (e.g. mmc_boot, usb_boot) are a strictly internal implementation
 detail and must not be used as a public interface.
 
-.. _BootLoaderSpec: http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/
+.. _`Boot Loader Specification`: https://systemd.io/BOOT_LOADER_SPECIFICATION/
 
 .. sectionauthor:: (C) Copyright 2014 Red Hat Inc.
 .. sectionauthor:: Copyright (c) 2014-2015, NVIDIA CORPORATION.  All rights reserved.