Recent changes

03. Booting Linux from the FIT image

There is a possibility that loading your FIP with the TechNexion fork of uuu will not work. If so, use the mainline uuu (that you've been using in the previous session) for booting your system and the fork from the previous task for flashing the eMMC.

Task A - List eMMC partition contents

First things first: let's check if the flashing process was succesful. Reload your FIP using uuu and get back into the U-Boot shell.

u-boot=> # list MMC devices (or rather, the slots)
u-boot=> mmc list
FSL_SDHC: 0 (eMMC)
FSL_SDHC: 1

u-boot=> # select MMC device 0
u-boot=> mmc dev 0
switch to partitions #0, OK
mmc0(part 0) is current device

u-boot=> # show partitions on currently selected MMC device
u-boot=> mmc part

Partition Map for MMC device 0  --   Partition Type: DOS

Part    Start Sector    Num Sectors     UUID            Type
  1     20480           184320          5a0f642d-01     83
  
u-boot=> # show contents of / directory on MMC device 0, partition 1
u-boot=> # assuming that the partition in question is FAT
u-boot=> fatls mmc 0:1
 72709475   linux.itb

1 file(s), 0 dir(s)

Task B - Load the FIT image in RAM

In order for U-Boot to parse the FIT image and extract its components at their respective load addresses, we first need to bring the entire FIT image in main memory. Choose a load address where the FIT image will not overlap with any of the three components when unpacked, or with the relocated bl33 code.

If fatls listed the contents of a FAT partition's directory, what can we use to load a file from said FAT partition? Seek help if in doubt.

Task C - Investigate contents of FIT image

This step is optional, but gives us the chance to see how U-Boot can parse a DTB file and extract its information. In the following example we assume that you've loaded the FIT image at address 0x 8000 0000 (i.e.: 2GB, 1GB offset in RAM):

u-boot=> # mark 0x 8000 0000 as the starting address of a device tree
u-boot=> fdt addr 0x80000000

u-boot=> # show contents of specific nodes in the RAM-based DTB
u-boot=> fdt list /
/ {
        timestamp = <0x64b55028>;
        description = "ASS - Linux FIT image for Pico Pi";
        #address-cells = <0x00000001>;
        images {
        };
        configurations {
        };
};

u-boot=> fdt list /configurations
configurations {
        default = "normal-boot";
        normal-boot {
        };
};

u-boot=> fdt list /configurations/normal-boot
normal-boot {
        description = "Normal boot config";
        kernel = "kernel";
        fdt = "fdt";
        ramdisk = "initrd";
};

Be careful not to list the contents of specific image nodes, such as /images/kernel. The fdt list command will print out the entire data attribute, meaning it will dump the entire kernel image to serial output.