**NOTE** I haven't yet tested direct boot through Raspberry's
         proprietary bootloader.

Booting Yggdrasil on Raspberry Pi 4B with u-boot:

1. Clone u-boot sources to some directory and checkout some
   stable branch. I've used v2024.10.
2. Modify cmd/boot.c by replacing the do_go_exec function:

/* Allow ports to override the default behavior */
__attribute__((weak))
unsigned long do_go_exec(ulong (*entry)(int, char * const []), int argc,
				 char *const argv[])
{
    void *entry_ptr = (void *) entry;
    ulong fdt_addr_r = 0;
    if (argc >= 2) {
        fdt_addr_r = hextoul(argv[1], NULL);
    }
    void (*func)(ulong) = entry_ptr;
    func(fdt_addr_r);

    return 0;
}

3. make CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm64 rpi_4_defconfig
4. make CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm64 -j
5. Copy u-boot.bin into your Pi SD-card's boot partition.
**NOTE** I assume you have all the bootloader parts in the boot partition already.
         If not, clone raspberry fw repo and copy the following files to the boot partition:
            * bootcode.bin
            * start4.elf
            * all the .dtb files (a bcm2711-rpi-4-b.dtb should be enough though)
6. config.txt:
enable_uart=1
arm64_bit=1
kernel=u-boot.bin

7. Compile the OS with `cargo xtask --arch=aarch64 --board=raspi4b --release`
8. Copy the following files into some directory:
    * target/aarch64-unknown-raspi4b/release/yggdrasil-kernel
    * userspace/target/aarch64-unknown-yggdrasil/release/initrd.tar
9. cd into that directory and start a TFTP server of your choice. I used `uftpd`.

10. Connect an ethernet and serial to the Pi and run the following commands in u-boot shell:

tftpboot 0x04000000 <YOUR IP>:initrd.tar
tftpboot ${loadaddr} <YOUR IP>:yggdrasil-kernel
load mmc 0:1 ${fdt_addr_r} bcm2711-rpi-4-b.dtb
fdt addr ${fdt_addr_r}
fdt resize
fdt memory 0x0 0x3C000000
fdt chosen 0x04000000 <WHATEVER SIZE WAS PRINTED WHEN RUNNING THE FIRST COMMAND>
bootelf -p
go ${kernel_addr_r} ${fdt_addr_r}

11. Yggdrasil OS should start!