diff options
Diffstat (limited to 'Documentation/arm')
| -rw-r--r-- | Documentation/arm/google/chromebook-boot-flow.rst | 69 | ||||
| -rw-r--r-- | Documentation/arm/index.rst | 2 | ||||
| -rw-r--r-- | Documentation/arm/tcm.rst | 2 |
3 files changed, 72 insertions, 1 deletions
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 diff --git a/Documentation/arm/index.rst b/Documentation/arm/index.rst index 2bda5461a80b..495ada7915e1 100644 --- a/Documentation/arm/index.rst +++ b/Documentation/arm/index.rst @@ -31,6 +31,8 @@ SoC-specific documents .. toctree:: :maxdepth: 1 + google/chromebook-boot-flow + ixp4xx marvell diff --git a/Documentation/arm/tcm.rst b/Documentation/arm/tcm.rst index b256f9783883..1dc6c39220f9 100644 --- a/Documentation/arm/tcm.rst +++ b/Documentation/arm/tcm.rst @@ -34,7 +34,7 @@ CPU so it is usually wise not to overlap any physical RAM with the TCM. The TCM memory can then be remapped to another address again using -the MMU, but notice that the TCM if often used in situations where +the MMU, but notice that the TCM is often used in situations where the MMU is turned off. To avoid confusion the current Linux implementation will map the TCM 1 to 1 from physical to virtual memory in the location specified by the kernel. Currently Linux |