doc: Add basic information about running CI tests

Start out by documenting general expectations on when CI is run, how
anyone can run Azure pipelines, and how GitLab CI pipelines can be run.

Signed-off-by: Tom Rini <trini@konsulko.com>

doc/develop/ci_testing.rst

@@ -0,0 +1,70 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Continuous Integration testing
+==============================
+
+All changes require passing our continuous integration tests prior to being
+merged in to mainline.  To help facilitate merges being accepted quickly,
+custodians are encouraged but not required to run a pipeline prior to sending a
+pull request.  Individual developers submitting significant or widespread
+changes are encouraged to run a pipeline themselves prior to posting.
+
+In order to make this process as easy as possible, the ability to run a CI
+pipeline is provided in both Azure and GitLab.  Both of these pipelines perform
+their Linux build jobs on the same Docker container image and to cover the same
+platforms.  In addition, Azure is also used to confirm that our host tools can
+be built with mingw to run on Windows.
+
+Each of the pipelines is written in such as way as to be a "world build" style
+test and as such we try and build all possible platforms.  In addition, for all
+platforms that support being run in QEMU we run them in QEMU and use our pytest
+suite.  See :doc:`py_testing` for more information about those tests.
+
+Azure Pipelines
+---------------
+
+This pipeline is defined in the top-level ``.azure-pipelines.yml`` file.
+Currently there are two ways to run a Microsoft Azure Pipeline test for U-Boot.
+
+The first way is to create an account with Microsoft at
+https://azure.microsoft.com/en-us/services/devops/ and then use the
+``.azure-pipelines.yml`` file in the U-Boot repository as the pipeline
+description.
+
+The second way is to use GitHub.  This requires a GitHub account
+and to fork the repository at https://github.com/u-boot/u-boot and to then
+submit a pull request as this will trigger an Azure pipeline run.  Clicking on
+your pull request on the list at https://github.com/u-boot/u-boot/pulls and
+then the "Checks" tab will show the results.
+
+GitLab CI Pipelines
+-------------------
+
+This pipeline is defined in the top-level ``.gitlab-ci.yml`` file.  Currently,
+we support running GitLab CI pipelines only for custodians, due to the
+resources the project has available.  For Custodians, it is a matter of
+enabling the pipeline feature in your project repository following the standard
+GitLab documentation.  For non-custodians, the pipeline itself is part of the
+tree and should be able to be used on any GitLab instance, with whatever
+runners you are able to provide.  While it is intended to be able to run this
+pipeline on the free public instances provided at https://gitlab.com/ a problem
+with our squashfs tests currently prevents this.
+
+Docker container
+----------------
+
+As previously stated, both of the above pipelines build using the same Docker
+container image.  This is maintained in the U-Boot source tree at
+``tools/docker/Dockerfile`` and new images are made as needed to support new
+tests or features.  This file needs to be updated whenever adding new external
+tool requirements to tests.
+
+Customizing CI
+--------------
+
+As noted above, the CI pipelines perform a world build.  While this is good for
+overall project testing, it can be less useful for testing specific cases or
+developing features.  In that case, it can be useful as part of your own
+testing cycle to edit these pipelines in separate local commits to pair them
+down to just the jobs you're interested in.  These changes must be removed
+prior to submission.

doc/develop/index.rst

@@ -9,6 +9,7 @@ Implementation
 .. toctree::
    :maxdepth: 1
 
+   ci_testing
    commands
    driver-model/index
    global_data