cmd: add support for a new "extension" command

This patch adds a new "extension" command, which aims at detecting
extension boards connected to the hardware platform, and apply the
Device Tree overlays that describe the hardware present on those
extension boards.

In order to enable this mechanism, board-specific code must implement
the extension_board_scan() function that fills in a linked list of
"struct extension", each describing one extension board. In addition,
the board-specific code must select the SUPPORT_EXTENSION_SCAN Kconfig
boolean.

Based on this:

 - "extension scan" makes the generic code call the board-specific
   extension_board_scan() function to retrieve the list of detected
   extension boards.

 - "extension list" allows to list the detected extension boards.

 - "extension apply <number>|all" allows to apply the Device Tree
   overlay(s) corresponding to one, or all, extension boards

The latter requires two environment variables to exist and set one variable
to run:

 - extension_overlay_addr: the RAM address where to load the Device
   Tree overlays

 - extension_overlay_cmd: the U-Boot command to load one overlay.
   Indeed, the location and mechanism to load DT overlays is very setup
   specific.

 - extension_overlay_name: set by the command: the name of the DT which
   will be load during the execution.

When calling the command described in the extension_overlay_cmd
variable, the variable extension_overlay_name will be defined. So a
typical extension_overlay_cmd will look like this:

  extension_overlay_cmd=load mmc 0:1 $extension_overlay_addr /boot/$extension_overlay_name

Here is an example on how to use it:
=> run loadfdt
=> fdt addr $fdtaddr
=> setenv extension_overlay_addr 0x1000
=> setenv extension_overlay_cmd 'load mmc 0:1 ${extension_overlay_addr} /boot/${extension_overlay_name}'
=> extension scan
Found 1 extension board(s).
=> extension apply 0
519 bytes read in 3 ms (168.9 KiB/s)

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>

1 files changed, 31 insertions, 0 deletions

diff --git a/include/extension_board.h b/include/extension_board.h
new file mode 100644
index 00000000000..c530a0ad8a2
--- /dev/null
+++ b/include/extension_board.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * (C) Copyright 2021
+ * Köry Maincent, Bootlin, <kory.maincent@bootlin.com>
+ */
+
+#ifndef __EXTENSION_SUPPORT_H
+#define __EXTENSION_SUPPORT_H
+
+struct extension {
+	struct list_head list;
+	char name[32];
+	char owner[32];
+	char version[32];
+	char overlay[32];
+	char other[32];
+};
+
+/**
+ * extension_board_scan - Add system-specific function to scan extension board.
+ * @param extension_list	List of extension board information to update.
+ * @return the number of extension.
+ *
+ * This function is called if CONFIG_CMD_EXTENSION is defined.
+ * Needs to fill the list extension_list with elements.
+ * Each element need to be allocated to an extension structure.
+ *
+ */
+int extension_board_scan(struct list_head *extension_list);
+
+#endif /* __EXTENSION_SUPPORT_H */