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--- ass:labs-2025:03:tasks:01 [2025/08/06 12:52]
florin.stancu
+++ ass:labs-2025:03:tasks:01 [2026/07/15 15:33] (current)
florin.stancu [01. Automatically booting our Linux]
@@ Line 2: / Line 2: @@
 You are probably be tired of using u-boot CLI to load the Linux FIT uImage into RAM and booting it manually.
-Surely, there must be something to be done to automate this... and you're right! Let's learn to do this!
+Surely, there must be something to be done to automate this (yep, there is!).
-First, boot your board into ''u-boot'' prompt. Enter ''env print''...
+First, let's learn about u-boot environment more... Start your board to a ''u-boot'' prompt and enter ''env print''...
 <spoiler env print output...>
@@ Line 101: / Line 101: @@
 First, let's create this file, let's say ''mydefault.env'' inside the uboot source directory. Let's start from a minimal example:
-<code mydefault.env>
+<code C>
 /* default u-boot environment variables */
 /* this file is passed through the C preprocessor (so we can use C-style macros!) */
@@ Line 114: / Line 114: @@
 console=ttyLP0
 bootargs=console=ttyLP0,115200 earlycon,115200 rdinit=/linuxrc clk_ignore_unused
-# This is the command executed automatically when uboot starts:
+# This is the command executed automatically when uboot starts...
-bootcmd=echo Fastboot mode... press Ctrl-C to exit; fastboot 0
+bootcmd=echo Fastboot mode... press Ctrl-C to exit; fastboot auto
 # This runs instead of bootcmd when booted using `uuu` via USB
 bootcmd_mfg=run bootcmd
-bootdelay=2
+bootdelay=3
-image=linux.itb
+# TODO: enter a valid uImage file DRAM load address (the one used with 'bootm')
 loadaddr=TODO
+# TODO: enter the 'load mmc 0 ...' script (will be executed using 'run loadimage')
 loadimage=TODO
+image=linux.itb
+# this can be executed using 'run linux' to boot your Linux kernel!
 linux=echo Booting Linux ...; run loadimage; bootm ${loadaddr};
 </code>
@@ Line 134: / Line 137: @@
 Enter your boot script inside the ''bootcmd'' var and let's proceed with overwriting the default environment.
-Recall the ''DEFAULT_ENV'' menuconfig item? Modify it to point to your ''mydefault.env'' (you can simply use a relative path). Note that you need to enable ''USE_DEFAULT_ENV_FILE'' checkbox first to let you supply your value!
+Recall the ''ENV_DEFAULT_ENV_TEXT_FILE'' menuconfig item? Modify it to point to your ''mydefault.env'' (you can simply use a relative path to ''u-boot''s source dir). Note that you need to enable ''ENV_USE_DEFAULT_ENV_TEXT_FILE'' checkbox first to let you modify that value!
-Afterwards, [re]compile u-boot, copy the u-boot ''.bin'' files again to the ''imx-mkimage'' directory and regenerate your ''flash.bin''.
+For advanced use cases / build automation, you can override any config items by creating extra .config files and using the included ''./scripts/kconfig/merge_config.sh'' u-boot host script!
-Test it by booting your new firmware using ''uuu''. Did it work? if not, you may need to repeat this process (this is where a script comes in handy!).
+Afterwards, [re]compile u-boot, copy the newly compiled u-boot ''.bin'' files to ''imx-mkimage'' directory again (if not doing it automatically using a Makefile) and regenerate your ''flash.bin''.
+Test it by booting your new firmware using ''uuu''.
+Always check the UBoot version and build time printed on the serial console during the boot process to ensure you loaded your latest firmware image!
+Try using ''run linux'' to run the ''linux'' script (if you used that), otherwise edit the ''bootcmd'' to do that automatically after the timeout!
+Did it work? if not, you may need to repeat this process (this is where a script comes in handy!).
+Note that real devices have their uboot configuration proceed automatically with booting the OS if a physical button is NOT pressed (remember those old Android phones? you could stop this process by holding several volume keys together!). You can script this using GPIO (try the ''gpio'' command!), but it's out of scope for today (we don't have any physical buttons).
+=== Bonus: load uboot environment from FAT32 partition ===
+If you wish to change without uboot's environment without recompiling u-boot itself, it's also possible (and very easy to do).
+Since this is usually desirable for flexibility during upgrades, there are many ways to do this: activate one of ''CONFIG_ENV_IS_IN_*'' config options or use a startup script which loads the env from a .txt file on the FAT32 partition. We'll prefer the latter since it's more fun (& failproof on most boards!).
+Basically, we will write a custom script to be executed by u-boot at startup. To do this, search for the ''PREBOOT'' config item inside u-boot's ''menuconfig''.
+Since we added a custom default.env file, we need to activate ''USE_PREBOOT'' and specify our script as a hardcoded ''preboot='' environment variable.
+Sample script:
+<code>
+preboot=if fatload mmc 0:1 ${loadaddr} uboot.txt; then env import -t ${loadaddr} ${filesize}; fi
+</code>
+That's it! Create this file on your board's FAT32 partition (using the same ''key=value'' assignments separated by newlines syntax) and boot it! You should now see your new environment values (''env print'')!