[PATCH v4 1/5] dt-bindings: Document how Chromebooks with depthcharge boot

[Douglas Anderson <dianders@chromium.org>]

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@chromium.org>
---
In my opinion this could land through the Qualcomm dts64 tree, mostly
because I want to land bindings patches in that tree that refer to
it. Since it's a new file it seems like there ought to be few
objections?

Changes in v4:
- Add reference to depthcharge and FIT Image.
- A few rst syntax fixups found by using a different rst preview.
- Updated wording as per Stephen.

Changes in v3:
- Fix up typos as per Matthias.
- Move under Documentation/arm/google/ as per Krzysztof.
- Add missing newline at end of file.

Changes in v2:
- ("Document how Chromebooks with depthcharge boot") new for v2.

 .../arm/google/chromebook-boot-flow.rst       | 69 +++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 Documentation/arm/google/chromebook-boot-flow.rst

diff --git a/Documentation/arm/google/chromebook-boot-flow.rst b/Documentation/arm/google/chromebook-boot-flow.rst
new file mode 100644
index 000000000000..36da77684bba
--- /dev/null
+++ b/Documentation/arm/google/chromebook-boot-flow.rst
@@ -0,0 +1,69 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================
+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.
+
+.. _depthcharge: https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/platform/depthcharge/
+.. _`FIT Image`: https://doc.coreboot.org/lib/payloads/fit.html
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2.36.1.124.g0e6072fb45-goog