[RFC 2/2] doc: add clang-format documentation

[Stephen Hemminger <stephen@networkplumber.org>]

Modify existing Linux kernel documentation of clang-format
to match usage in DPDK.

Still waiting for right to reuse acknowledgment for original
Linux kernel author of this documentation.  If not ok, then
can just write a new version.

Probably need some wording about applicability to base
driver code.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 doc/guides/contributing/clang-format.rst | 184 +++++++++++++++++++++++
 doc/guides/contributing/index.rst        |   1 +
 2 files changed, 185 insertions(+)
 create mode 100644 doc/guides/contributing/clang-format.rst

diff --git a/doc/guides/contributing/clang-format.rst b/doc/guides/contributing/clang-format.rst
new file mode 100644
index 000000000000..4b3dda0d520c
--- /dev/null
+++ b/doc/guides/contributing/clang-format.rst
@@ -0,0 +1,184 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright 2023 The DPDK contributors
+    Original from Linux kernel docs
+
+.. _clangformat:
+
+clang-format
+============
+
+``clang-format`` is a tool to format C/C++/... code according to
+a set of rules and heuristics. Like most tools, it is not perfect
+nor covers every single case, but it is good enough to be helpful.
+
+``clang-format`` can be used for several purposes:
+
+  - Quickly reformat a block of code to the DPDK style. Specially useful
+    when moving code around and aligning/sorting. See clangformatreformat_.
+
+  - Spot style mistakes, typos and possible improvements in files
+    you maintain, patches you review, diffs, etc. See clangformatreview_.
+
+  - Help you follow the coding style rules, specially useful for those
+    new to DPDK development or working at the same time in several
+    projects with different coding styles.
+
+Its configuration file is ``.clang-format`` in the root of the DPDK tree.
+The rules contained there try to approximate the most common DPDK
+coding style. They also try to follow :ref:`coding_style`
+as much as possible. A driver may have a slightly different style,
+it is possible that you may want to tweak the defaults for a particular
+folder. To do so, you can override the defaults by writing
+another ``.clang-format`` file in a subfolder.
+
+The tool itself has already been included in the repositories of popular
+Linux distributions for a long time. Search for ``clang-format`` in
+your repositories. Otherwise, you can either download pre-built
+LLVM/clang binaries or build the source code from:
+
+    https://releases.llvm.org/download.html
+
+See more information about the tool at:
+
+    https://clang.llvm.org/docs/ClangFormat.html
+
+    https://clang.llvm.org/docs/ClangFormatStyleOptions.html
+
+
+.. _clangformatreview:
+
+Review files and patches for coding style
+-----------------------------------------
+
+By running the tool in its inline mode, you can review full subsystems,
+folders or individual files for code style mistakes, typos or improvements.
+
+To do so, you can run something like::
+
+    # Make sure your working directory is clean!
+    clang-format -i driver/*.[ch]
+
+And then take a look at the git diff.
+
+Counting the lines of such a diff is also useful for improving/tweaking
+the style options in the configuration file; as well as testing new
+``clang-format`` features/versions.
+
+``clang-format`` also supports reading unified diffs, so you can review
+patches and git diffs easily. See the documentation at:
+
+    https://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting
+
+To avoid ``clang-format`` formatting some portion of a file, you can do::
+
+    int formatted_code;
+    // clang-format off
+        void    unformatted_code  ;
+    // clang-format on
+    void formatted_code_again;
+
+While it might be tempting to use this to keep a file always in sync with
+``clang-format``, specially if you are writing new files or if you are
+a maintainer, please note that people might be running different
+``clang-format`` versions or not have it available at all. Therefore,
+you should probably refrain yourself from using this for all sources.
+
+.. _clangformatreformat:
+
+Reformatting blocks of code
+---------------------------
+
+By using an integration with your text editor, you can reformat arbitrary
+blocks (selections) of code with a single keystroke. This is specially
+useful when moving code around, for complex code that is deeply intended,
+for multi-line macros (and aligning their backslashes), etc.
+
+Remember that you can always tweak the changes afterwards in those cases
+where the tool did not do an optimal job. But as a first approximation,
+it can be very useful.
+
+There are integrations for many popular text editors. For some of them,
+like vim, emacs, BBEdit and Visual Studio you can find support built-in.
+For instructions, read the appropriate section at:
+
+    https://clang.llvm.org/docs/ClangFormat.html
+
+For Atom, Eclipse, Sublime Text, Visual Studio Code, XCode and other
+editors and IDEs you should be able to find ready-to-use plugins.
+
+For this use case, consider using a secondary ``.clang-format``
+so that you can tweak a few options. See clangformatextra_.
+
+
+.. _clangformatmissing:
+
+Missing support
+---------------
+
+``clang-format`` is missing support for some things that are common
+in DPDK code. They are easy to remember, so if you use the tool
+regularly, you will quickly learn to avoid/ignore those.
+
+In particular, some very common ones you will notice are:
+
+  - Aligned blocks of one-line ``#defines``, e.g.::
+
+        #define TRACING_MAP_BITS_DEFAULT       11
+        #define TRACING_MAP_BITS_MAX           17
+        #define TRACING_MAP_BITS_MIN           7
+
+    vs.::
+
+        #define TRACING_MAP_BITS_DEFAULT 11
+        #define TRACING_MAP_BITS_MAX 17
+        #define TRACING_MAP_BITS_MIN 7
+
+  - Aligned designated initializers, e.g.::
+
+        static const struct eth_dev_ops ops = {
+                .dev_close     = eth_dev_close,
+	        .dev_start     = eth_dev_start,
+	        .dev_stop      = eth_dev_stop,
+	        .dev_configure = eth_dev_configure,
+                .dev_infos_get = eth_dev_info,
+        };
+
+    vs.::
+
+        static const struct eth_dev_ops ops = {
+                .dev_close = eth_dev_close,
+	        .dev_start = eth_dev_start,
+	        .dev_stop = eth_dev_stop,
+	        .dev_configure = eth_dev_configure,
+                .dev_infos_get = eth_dev_info,
+        };
+
+
+.. _clangformatextra:
+
+Extra features/options
+----------------------
+
+Some features/style options are not enabled by default in the configuration
+file in order to minimize the differences between the output and the current
+code. In other words, to make the difference as small as possible,
+which makes reviewing full-file style, as well diffs and patches as easy
+as possible.
+
+In other cases (e.g. particular subsystems/folders/files), the DPDK style
+might be different and enabling some of these options may approximate
+better the style there.
+
+For instance:
+
+  - Aligning assignments (``AlignConsecutiveAssignments``).
+
+  - Aligning declarations (``AlignConsecutiveDeclarations``).
+
+  - Reflowing text in comments (``ReflowComments``).
+
+  - Sorting ``#includes`` (``SortIncludes``).
+
+They are typically useful for block re-formatting, rather than full-file.
+You might want to create another ``.clang-format`` file and use that one
+from your editor/IDE instead.
diff --git a/doc/guides/contributing/index.rst b/doc/guides/contributing/index.rst
index 7a9e6b368e1c..68e70f2350a4 100644
--- a/doc/guides/contributing/index.rst
+++ b/doc/guides/contributing/index.rst
@@ -9,6 +9,7 @@ Contributor's Guidelines
     :numbered:
 
     coding_style
+    clang-format
     design
     abi_policy
     abi_versioning
-- 
2.39.2