CODEWITHSUNDEEP
linux-kernelraspberry-piu-bootdevice-tree
user8908459
user8908459

Reputation: 577

How does the raspbery pi know which device tree blob (dtb) to load?

A raspberry pi card images includes several different dtb files in the boot partition. For example:

bcm2708-rpi-0-w.dtb
bcm2708-rpi-b.dtb
bcm2708-rpi-b-plus.dtb
bcm2708-rpi-cm.dtb
bcm2709-rpi-2-b.dtb
bcm2710-rpi-3-b.dtb
bcm2710-rpi-3-b-plus.dtb
bcm2710-rpi-cm3.dtb

When the pi boots, how does it know when dtb to select?

Upvotes: 0

Views: 5185

Answers (2)

user5395338
user5395338

Reputation:

I'm not sure the currently accepted answer is correct for Raspberry Pi OS - at least the "official documentation" explains it quite differently. It's further unclear why u-boot was part of the answer as Raspberry Pi OS doesn't use u-boot; and the OP didn't mention u-boot.

I'm not "quality police", and claim no expertise in this area - this Q&A was listed in a search result, and it seemed a bit off... thus, this answer.

And finally - to an answer: These various .dtb files contain the "hardware definitions" of the various hardware models of Raspberry Pi; you can see the model designation in the filename; e.g. bcm2710-rpi-3-b-plus.dtb designates an RPi 3B+. The .dtb extension is shorthand for "device tree blob/binary" - compiled from a .dts "device tree source" file. The hardware model is detected during the boot process, and is used to select which .dtb file to use, which in turn is used to set up the kernel. The Rpi hardware model designation may be found on any RPi as follows:

cat /proc/cpuinfo | grep Model

The "official" documentation has a much more detailed answer. The documentation also has more details on device trees in general if you're interested.

Upvotes: 3

Xypron
Xypron

Reputation: 2483

In U-Boot file board/raspberrypi/rpi/rpi.c, function get_board_rev() the board revision is read and the dtb name is determined from the revision. The dtb file name is set in the environment variable ftdfile which can be used in a boot script to load the appropriate file which then is passed on to the operating system.

Upvotes: 2

Related Questions