Chromebook Boot Flow

Most recent Chromebooks that use device tree are using the opensource depthcharge bootloader. Depthcharge expects the OS to be packaged as a FIT Image which contains an OS image as well as a collection of device trees. It is up to depthcharge to pick the right device tree from the FIT Image and provide it to the OS.

The scheme that depthcharge uses to pick the device tree takes into account three variables:

  • Board name, specified at depthcharge compile time. This is $(BOARD) below.

  • Board revision number, determined at runtime (perhaps by reading GPIO strappings, perhaps via some other method). This is $(REV) below.

  • SKU number, read from GPIO strappings at boot time. This is $(SKU) below.

For recent Chromebooks, depthcharge creates a match list that looks like this:

  • google,$(BOARD)-rev$(REV)-sku$(SKU)

  • google,$(BOARD)-rev$(REV)

  • google,$(BOARD)-sku$(SKU)

  • google,$(BOARD)

Note that some older Chromebooks use a slightly different list that may not include SKU matching or may prioritize SKU/rev differently.

Note that for some boards there may be extra board-specific logic to inject extra compatibles into the list, but this is uncommon.

Depthcharge will look through all device trees in the FIT Image trying to find one that matches the most specific compatible. It will then look through all device trees in the FIT Image trying to find the one that matches the second most specific compatible, etc.

When searching for a device tree, depthcharge doesn’t care where the compatible string falls within a device tree’s root compatible string array. As an example, if we’re on board “lazor”, rev 4, SKU 0 and we have two device trees:

  • “google,lazor-rev5-sku0”, “google,lazor-rev4-sku0”, “qcom,sc7180”

  • “google,lazor”, “qcom,sc7180”

Then depthcharge will pick the first device tree even though “google,lazor-rev4-sku0” was the second compatible listed in that device tree. This is because it is a more specific compatible than “google,lazor”.

It should be noted that depthcharge does not have any smarts to try to match board or SKU revisions that are “close by”. That is to say that if depthcharge knows it’s on “rev4” of a board but there is no “rev4” device tree then depthcharge won’t look for a “rev3” device tree.

In general when any significant changes are made to a board the board revision number is increased even if none of those changes need to be reflected in the device tree. Thus it’s fairly common to see device trees with multiple revisions.

It should be noted that, taking into account the above system that depthcharge has, the most flexibility is achieved if the device tree supporting the newest revision(s) of a board omits the “-rev{REV}” compatible strings. When this is done then if you get a new board revision and try to run old software on it then we’ll at pick the newest device tree we know about.