how do cape overlays work

RobertCNelson · January 20, 2022, 7:40pm

Everything should be done in u-boot:

cmd: add support for a new "extension" command

committed 05:09PM - 13 May 21 UTC

This patch adds a new "extension" command, which aims at detecting extension boa…rds 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>

Nothing should be done “while” the kernel is loading…

Regards,