getting u-boot to work on raspi 3b (32bit)

[Simon Glass]

Hi Robert,

On Tue, 25 Jul 2023 at 09:42, Robert Wenisch <RWenisch at bfs.de> wrote:
>
> Hello,
>
> in order to study the boot process of embedded ARM systems, I decided to start with something well documented: booting as raspi 3b using u-boot.
> I downloaded kernel sources and u-boot sources. I built the kernel with
>
> CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm make bcmrpi_defconfig
> CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm make -j6 zImage dtbs modules
> cp arch/arm/boot/zImage /path/to/sdcard-boot-partition
> cp arch/arm/boot/dts/bcm2710-rpi-3-b.dtb /path/to/sdcard-boot-partition
> CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm INSTALL_MOD_PATH make modules_install
>
> Further I compiled and setup u-boot:
>
> CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm make rpi_3_32b_defconfig
> CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm make -j6
>
> cp u-boot.bin /path/to/ sdcard-boot-partition
>
> I then went on to edit the config.txt:
>
> kernel=u-boot-bin
> enable_uart=1
> arm_64bit=0
> device_tree=bcm2710-rpi-3-b.dtb
>
> and boot.txt goes
>
> fatload mmc 0:1 ${fdt_addr_r} bcm2710-rpi-3-b.dtb
> fatload mmc 0:1 $kernel_addr_r} zImage
> setenv bootargs console=ttyS0,115200 root=/dev/mmcblk0p2 rootfstype=ext4 rootwait rw earlyprintk
> bootz ${kernel_addr_r} - {fdt_addr_r}
>
> I produced an image of boot.txt
>
> mkimage -A arm -O linux -T script -d boot.txt boot.scr
>
> Frustratingly, it didn't go as smoothly as expected.

You should really be using FIT [1] for this, rather than legacy images.

>
> With these setting I got up to state "Starting Kernel ..." over the serial port, then it freezes (or at least doesn't log anything on the tty).
> I can't post the whole log as I only have it on my dev-machine.

You could try things like earlycon to get some console output.

> However some interesting bits for now:
> I'm booting on u-boot 2023-07-0967-g94e7cb181a
> 1 - Pretty much at the beginning it says "Loading Environment from FAT... *** warning - bad CRC, using default environment"
> I suppose this only means we're not providing a .env file?

Yes

>
> 2 - Upon "Scanning bus usb at 7e980000 for devices... it's logging a lot of garbage (as in misinterpreted character codes), what might be going on there?

No...some sort of clock problem?
>
> 3 - A bit further down it states "Found U-Boot script /boot.scr"
>  [...]
> "## Executing script at 02400000"
> As this changes when I edit around the boot.txt and mkimage, I infer u-boot is indeed loading my boot instructions not just some default values.

OK

>
> Then follow some info on kernel image flatted device tree blob and their respective memory addresses.
> It the tries to load the kernel to little apparent success :/
>
> This procedure is what I gathered from a plethora of tutorial and howtos. Most of these are quite old, have there been breaking changes in u-boot's development in the meanwhile?
> For example some sources state the kernel is launched via booti ${kernel_addr_r} - {fdt_addr_r}, however the booti command seem to be absent in my u-boot version.

I believe booti is for arm64 but you are using the 32-bit build of
rpi-3, so you should be using bootm.

>
> Is there anything else I#m doing wrong?
>
> I'll be grateful for any hints at this point...

You could send a patch to [1] to help others!

There are also distros like Armbian which you can use out of the box.

Regards,
Simon

[1] https://u-boot.readthedocs.io/en/latest/usage/fit/index.html
[2] https://u-boot.readthedocs.io/en/latest/board/broadcom/raspberrypi.html