Enable to support USB boot

docs/extra-build-config.md

@@ -74,6 +74,22 @@ To remove (or adjust) this delay set these variables in local.conf:
     BOOT_DELAY = "0"
     BOOT_DELAY_MS = "0"
 
+## Boot media
+
+The Raspberry Pi 4 board can load the boot image files from SD card and USB memory.
+By default SD card media is used as boot media.
+
+To switch the boot media from SD card to USB memory, the following variables are supported
+in local.conf: `CMDLINE_ROOT_PARTITION` and `BOOT_MEDIA`.
+The default value of `CMDLINE_ROOT_PARTITION` is "/dev/mmcblk0p2" to mount SD card. If you want to mount USB memory partition, set CMDLINE_ROOT_PARTITION to "/dev/sda2".
+`BOOT_MEDIA` allows  `mmc` and `usb`. The "mmc" is required to load an image from the SD card, following the u-boot specification. Similarly, if you want to load a boot image file from USB memory, set BOOT_MEDIA to "usb".
+
+For example, if you want to use USB boot, please define
+the following parameters in your local.conf file.
+
+    CMDLINE_ROOT_PARTITION = "/dev/sda2"
+    BOOT_MEDIA = "usb"
+
 ## Set overclocking options
 
 The Raspberry Pi can be overclocked. As of now overclocking up to the "Turbo

recipes-bsp/bootfiles/rpi-cmdline.bb

@@ -9,7 +9,9 @@ inherit deploy nopackages
 CMDLINE_DWC_OTG ?= "dwc_otg.lpm_enable=0"
 
 CMDLINE_ROOT_FSTYPE ?= "rootfstype=ext4"
-CMDLINE_ROOTFS ?= "root=/dev/mmcblk0p2 ${CMDLINE_ROOT_FSTYPE} rootwait"
+CMDLINE_ROOT_PARTITION ?= "/dev/mmcblk0p2"
+
+CMDLINE_ROOTFS ?= "root=${CMDLINE_ROOT_PARTITION} ${CMDLINE_ROOT_FSTYPE} rootwait"
 
 CMDLINE_SERIAL ?= "${@oe.utils.conditional("ENABLE_UART", "1", "console=serial0,115200", "", d)}"
 

recipes-bsp/rpi-u-boot-scr/files/boot.cmd.in

@@ -1,4 +1,4 @@
 fdt addr ${fdt_addr} && fdt get value bootargs /chosen bootargs
-fatload mmc 0:1 ${kernel_addr_r} @@KERNEL_IMAGETYPE@@
-if test ! -e mmc 0:1 uboot.env; then saveenv; fi;
+fatload @@BOOT_MEDIA@@ 0:1 ${kernel_addr_r} @@KERNEL_IMAGETYPE@@
+if test ! -e @@BOOT_MEDIA@@ 0:1 uboot.env; then saveenv; fi;
 @@KERNEL_BOOTCMD@@ ${kernel_addr_r} - ${fdt_addr}

recipes-bsp/rpi-u-boot-scr/rpi-u-boot-scr.bb

@@ -9,9 +9,12 @@ INHIBIT_DEFAULT_DEPS = "1"
 
 SRC_URI = "file://boot.cmd.in"
 
+BOOT_MEDIA ?= "mmc"
+
 do_compile() {
     sed -e 's/@@KERNEL_IMAGETYPE@@/${KERNEL_IMAGETYPE}/' \
         -e 's/@@KERNEL_BOOTCMD@@/${KERNEL_BOOTCMD}/' \
+        -e 's/@@BOOT_MEDIA@@/${BOOT_MEDIA}/' \
         "${WORKDIR}/boot.cmd.in" > "${WORKDIR}/boot.cmd"
     mkimage -A ${UBOOT_ARCH} -T script -C none -n "Boot script" -d "${WORKDIR}/boot.cmd" boot.scr
 }