README.md: update instructions with octopus example

Update the instructions about how to use the tools in this repository
and provide an example based on octopus.  This includes setting up a
Docker container and building a firmware image.
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>

README.md

@@ -47,24 +47,89 @@ SUCCESS
 # Building
 
 The Chromebook firmware needs to be rebuilt and flashed with extra patches and
-configuration options turned on in order to enable the serial console in the
-bootloader and boot with a kernel image and ramdisk supplied over TFTP
-interactively.  The standard firmware shipped with the products is only
-configured to boot the Chrome OS image present on the device, with no serial
-console and no way to override it.  This is not suitable for automating the
-Chromebook in a test lab such as LAVA, or for doing low-level kernel
-development.
+configuration options turned on in order to enable the serial console and boot
+with a kernel image and ramdisk supplied over TFTP interactively.  The standard
+firmware shipped with the products is only configured to boot the Chrome OS
+image present on the device, with no serial console and no way to override it.
+This is not suitable for automating the Chromebook in a test lab such as LAVA,
+or for doing low-level kernel development.
+
+Collabora maintains a set of
+[`Depthcharge`](https://gitlab.collabora.com/chromium/depthcharge/) branches
+with such changes.  The tools in this repository make use of them to build the
+firmware images used in LAVA.
+
+## Docker containers
 
 Building the firmware for a Chromebook can be non-trivial.  To make things
-easier, this repository contains a Docker container and some helper scripts to
-build firmware images for some known device types.
+easier, this repository provides tools to create Docker containers and some
+helper scripts to build firmware images for some known device types.  All this is kept under the [`cros-build`](https://gitlab.collabora.com/chromium/firmware-tools/-/blob/master/cros-build/) directory.
+
+The containers are using the following local directories on the host:
+
+* cache: used by the SDK tools, for example to store SDK tarballs
+* firmware: where the firmware binaries are kept and new ones are placed
+* chroot-*: dedicated chroot directory for each device type
+
+## Chrombeook device types
+
+Each device type uses a different revision of the Chromium OS source tree,
+which means a different Docker image and a different Docker container with a
+different chroot directory.  Each device type will also have a different build
+script, as the steps can vary slightly for each of them.
 
 The
 [`cros-build/bootstrap.sh`](https://gitlab.collabora.com/chromium/firmware-tools/-/blob/master/cros-build/bootstrap.sh)
-script can be used to create local directories, then build the Docker container
-which will be using them and start it.  It takes only one argument which is the
-device type.  For example:
+script can be used to create a Docker container for a particular device type.
+It will download the Chromium OS source code necessary to build a Chromebook
+firmware and set up the Chromium OS SDK chroot.  Then it's possible to run some
+build scripts provided in this repository to build a new image.
+
+All these device-specific things can be found in the
+[`setup`](https://gitlab.collabora.com/chromium/firmware-tools/-/blob/master/cros-build/setup/)
+sub-directory.  See the example below for the `octopus` device type.
+
+## Example: octopus
+
+For example, the `octopus` device type has a
+[`octopus.env`](https://gitlab.collabora.com/chromium/firmware-tools/-/blob/master/cros-build/setup/octopus.env)
+file with environment variables defining the parameters for the Docker
+container and a
+[`octopus.sh`](https://gitlab.collabora.com/chromium/firmware-tools/-/blob/master/cros-build/setup/octopus.sh)
+build script which will get copied in the container.
+
+To set up a container for `octopus`:
 
 ```
+cd cros-build
 ./bootstrap.sh octopus
+Using environment file: setup/octopus.env
+------------------------
+CROS_DEVICE=octopus
+CROS_SDK_BRANCH=firmware-octopus-11297.83.B
+------------------------
+[...]
+(cr) (firmware-octopus-11297.83.B/(8c78090...)) cros-build@4d71a209fc9f ~/trunk/src/scripts $
+```
+
+This can take a while the first time.  Once it has completed, exiting from the
+container and running the `bootstrap.sh` script again should only take a few
+seconds as everything is kept in the `cache` and `chroot-octopus` directories.
+
+Then to start building the firmware, the `octopus.sh` script is available
+
+```
+./octopus.sh setup    # to configure the chroot for "octopus"
+./octopus.sh checkout # to check out the Depthcharge branch
+./octopus.sh build    # to build Depthcharge
+./octopus.sh image    # to create a new firmware image
+```
+
+Likewise, the `setup` and `build` steps can take a while the first time but
+should be very quick when run again in the same chroot.
+
+If everything went well, there should be a new firmware image:
+```
+-rw-r--r-- 1 cros-build chronos 16777216 Oct 21 10:20 firmware/octopus-new.bin
 ```
+This can be accessed from the host, in the `firmware` directory.