This documents how many Chromebooks pick the device tree that will be
passed to the OS and can help understand the revisions / skus listed
as the top-level "compatible" in many Chromebooks.
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
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Changes in v2:
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+.. SPDX-License-Identifier: GPL-2.0
+
+======================================
+Chromebook Boot Flow
+======================================
+
+Most recent Chromebooks that use device tree boot 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 compile time.
+- Board revision number, read from GPIO strappings at boot time.
+- SKU number, read from GPIO strappings at boot time.
+
+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 may 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 falls within a given device tree. 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 that 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 the
+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 most
+reasonable device tree. If it turns out that the new revision actually
+has no device-tree visible changes then we'll not only pick the most
+reasonable device tree, we'll pick the exact right one.
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2.36.0.550.gb090851708-goog
