[v2,1/4] dt-bindings: Document how Chromebooks with depthcharge boot

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>
---

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

 .../devicetree/chromebook-boot-flow.rst       | 63 +++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100644 Documentation/devicetree/chromebook-boot-flow.rst

On 13/05/2022 18:59, Douglas Anderson wrote:
> 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>
> ---
> 
> Changes in v2:
> - ("Document how Chromebooks with depthcharge boot") new for v2.
> 
>  .../devicetree/chromebook-boot-flow.rst       | 63 +++++++++++++++++++
>  1 file changed, 63 insertions(+)
>  create mode 100644 Documentation/devicetree/chromebook-boot-flow.rst

This is not a Devicetree (generic) document, so it is not the best place
for it. Maybe regular vendor place, e.g . Documentation/arm/google/ ?

> 
> diff --git a/Documentation/devicetree/chromebook-boot-flow.rst b/Documentation/devicetree/chromebook-boot-flow.rst
> new file mode 100644
> index 000000000000..84aeb0a17ee4
> --- /dev/null
> +++ b/Documentation/devicetree/chromebook-boot-flow.rst
> @@ -0,0 +1,63 @@
> +.. 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.
> \ No newline at end of file

Missing new line.


Best regards,
Krzysztof