112 lines
2.9 KiB
ReStructuredText
112 lines
2.9 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
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.. Copyright 2021, Kory Maincent <kory.maincent@bootlin.com>
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U-Boot extension board usage (CONFIG_EXTENSION)
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===============================================
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Synopsis
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--------
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::
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extension scan
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extension list
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extension apply <extension number|all>
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Description
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-----------
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The "extension" command proposes a generic U-Boot mechanism to detect
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extension boards connected to the HW platform, and apply the appropriate
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Device Tree overlays depending on the detected extension boards.
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The "extension" command comes with three sub-commands:
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- "extension scan" makes the generic code call the board-specific
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extension_board_scan() function to retrieve the list of detected
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extension boards.
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- "extension list" allows to list the detected extension boards.
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- "extension apply <number>|all" allows to apply the Device Tree
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overlay(s) corresponding to one, or all, extension boards
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The latter requires two environment variables to exist:
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- extension_overlay_addr: the RAM address where to load the Device
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Tree overlays
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- extension_overlay_cmd: the U-Boot command to load one overlay.
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Indeed, the location and mechanism to load DT overlays is very setup
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specific.
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In order to enable this mechanism, board-specific code must implement
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the extension_board_scan() function that fills in a linked list of
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"struct extension", each describing one extension board. In addition,
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the board-specific code must select the SUPPORT_EXTENSION_SCAN Kconfig
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boolean.
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Usage example
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-------------
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1. Make sure your devicetree is loaded and set as the working fdt tree.
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::
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=> run loadfdt
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=> fdt addr $fdtaddr
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2. Prepare the environment variables
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::
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=> setenv extension_overlay_addr 0x88080000
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=> setenv extension_overlay_cmd 'load mmc 0:1 ${extension_overlay_addr} /boot/${extension_overlay_name}'
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3. Detect the plugged extension board
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::
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=> extension scan
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4. List the plugged extension board information and the devicetree
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overlay name
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::
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=> extension list
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5. Apply the appropriate devicetree overlay
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For apply the selected overlay:
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::
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=> extension apply 0
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For apply all the overlays:
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::
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=> extension apply all
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Simple extension_board_scan function example
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--------------------------------------------
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.. code-block:: c
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int extension_board_scan(struct list_head *extension_list)
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{
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struct extension *extension;
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extension = calloc(1, sizeof(struct extension));
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snprintf(extension->overlay, sizeof(extension->overlay), "overlay.dtbo");
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snprintf(extension->name, sizeof(extension->name), "extension board");
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snprintf(extension->owner, sizeof(extension->owner), "sandbox");
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snprintf(extension->version, sizeof(extension->version), "1.1");
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snprintf(extension->other, sizeof(extension->other), "Extension board information");
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list_add_tail(&extension->list, extension_list);
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return 1;
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}
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