U-Boot extension board usage (CONFIG_EXTENSION)


extension scan
extension list
extension apply <extension number|all>


The “extension” command proposes a generic U-Boot mechanism to detect extension boards connected to the HW platform, and apply the appropriate Device Tree overlays depending on the detected extension boards.

The “extension” command comes with three sub-commands:

  • “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:

  • 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.

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.

Usage example

  1. Make sure your devicetree is loaded and set as the working fdt tree.
=> run loadfdt
=> fdt addr $fdtaddr
  1. Prepare the environment variables
=> setenv extension_overlay_addr 0x88080000
=> setenv extension_overlay_cmd 'load mmc 0:1 ${extension_overlay_addr} /boot/${extension_overlay_name}'
  1. Detect the plugged extension board
=> extension scan
  1. List the plugged extension board information and the devicetree overlay name
=> extension list
  1. Apply the appropriate devicetree overlay

For apply the selected overlay:

=> extension apply 0

For apply all the overlays:

=> extension apply all

Simple extension_board_scan function example

int extension_board_scan(struct list_head *extension_list)
    struct extension *extension;

    extension = calloc(1, sizeof(struct extension));
    snprintf(extension->overlay, sizeof(extension->overlay), "overlay.dtbo");
    snprintf(extension->name, sizeof(extension->name), "extension board");
    snprintf(extension->owner, sizeof(extension->owner), "sandbox");
    snprintf(extension->version, sizeof(extension->version), "1.1");
    snprintf(extension->other, sizeof(extension->other), "Extension board information");
    list_add_tail(&extension->list, extension_list);

    return 1;