Got to boot a RPi Zero 2 W with Debian

About a month ago, I got tired of waiting for the newest member of the Raspberry product lineup to be sold in Mexico, and I bought it from a Chinese reseller through a big online shopping platform. I paid quite a bit of premium (~US$85 instead of the advertised US$15), and got it delivered ten days later…

Anyway, it’s known this machine does not yet boot mainline Linux. The vast majority of ARM systems require the bootloader to load a Device Tree file, presenting the hardware characteristics map. And while the RPi Zero 2 W (hey… what an awful and confusing naming scheme they chose!) is mostly similar to a RPi3B+, it is not quite the same thing. A kernel with RPi3B+’s device tree will refuse to boot.

Anyway, I started digging, and found that some days ago Stephan Wahren sent a patch to the linux-arm-kernel mailing list with a matching device tree. Read the patch! It’s quite simple to read (what is harder is to know where each declaration should go, if you want to write your own, of course). It basically includes all basic details for the main chip in the RPi3 family (BCM2837), pulls in also the declarations from the BCM2836 present in the RPi2, and adds the necessary bits for the USB OTG connection and the WiFi and Bluetooth declarations. Registers the model name as Raspberry Pi Zero 2 W, which you can easily see in the following photo, informs the kernel it has 512MB RAM, and… Well, really, it’s an easy device tree to read, don’t be shy!

So, I booted my RPi 3B+ with a freshly downloaded Bookworm image, installed and unpacked linux-source-5.15, applied Stephan’s patch, and added the following for the DTB to be generated in the arm64 tree as well:

--- /dev/null   2022-01-26 23:35:40.747999998 +0000
+++ arch/arm64/boot/dts/broadcom/bcm2837-rpi-zero-2-w.dts       2022-02-13 06:28:29.968429953 +0000
@@ -0,0 +1 @@
+#include "arm/bcm2837-rpi-zero-2-w.dts"

Then, ran a simple make dtbs, and… Failed, because bcm283x-rpi-wifi-bt.dtsi is not yet in the kernel 😦.

OK, no worries: Getting wireless to work is a later step. I commented out the lines causing conflict (10, 33-35, 134-136), and:

root@rpi3-20220212:/usr/src/linux-source-5.15# make dtbs
  DTC     arch/arm64/boot/dts/broadcom/bcm2837-rpi-zero-2-w.dtb

Great!

Just copied over that generated file to /boot/firmware/, moved the SD over to my RPiZ2W, and behold! It boots!

When I bragged about it in #debian-raspberrypi, steev suggested me to pull in the WiFi patch, that has also been submitted (but not yet accepted) for kernel inclusion. I did so, uncommented the lines I modified, and built again. It builds correctly, and –again– copied the DTB over. It still does not find the WiFi; dmesg still complains about a missing bit of firmware (failed to load brcm/brcmfmac43430b0-sdio.raspberrypi,model-zero-2-w.bin). Steev pointed out it can be downloaded from RPi Distro’s GitHub page, but I called it a night and didn’t pursue it any further ;-)

So… I understand this post is still a far cry from saying “our images properly boot under a RPi 0 2 W”, but… we will get there 😉