[RFC PATCH v2] treewide: add .clang-format configuration file

[Rahul Sandhu <nvraxn@gmail.com>]

Currently only an RFC.

Add the .clang-format configuration file, taken from the Linux kernel
repository. We don't have any official style guidelines in tree at
present, which makes it a bit unclear how to format C code for new
contributors. As well as this, different parts of the codebase seem to
been formatted with different styles on occasion, so using an automatic
formatter should resolve this.

.clang-format is also read by various editors and tooling for writing C
code. It may also be worth adding an editorconfig file in the future as
well?

It may well also be worth adding a `format` target to the Makefiles, is
that something that would be useful?

A few other things to consider to do in the future:
1. Reformat all existing code. I understand this is a big change, hence
   the RFC, but we may as well get all code formatted if we go down
   this route; afterall, it's not like this will cause any breaking
   changes.
2. Possibly add a CI target to check that all code is formatted as per
   the new clang-format configuration? The options `--dry-run` as well
   as `-Werror` can be passed to clang-format for this.

Comments/feedback appreciated, thanks.

Signed-off-by: Rahul Sandhu <nvraxn@gmail.com>
---
 .clang-format   | 130 ++++++++++++++++++++++++++++++++++++++++++++++++
 CONTRIBUTING.md |   2 +
 2 files changed, 132 insertions(+)
 create mode 100644 .clang-format

v2: remove linux kernel ForEachMacros and replace them with ours

diff --git a/.clang-format b/.clang-format
new file mode 100644
index 00000000..35595d87
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,130 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# clang-format configuration file. Intended for clang-format >= 11.
+#
+# For more information, see:
+#
+#   Documentation/dev-tools/clang-format.rst
+#   https://clang.llvm.org/docs/ClangFormat.html
+#   https://clang.llvm.org/docs/ClangFormatStyleOptions.html
+#
+---
+AccessModifierOffset: -4
+AlignAfterOpenBracket: Align
+AlignConsecutiveAssignments: false
+AlignConsecutiveDeclarations: false
+AlignEscapedNewlines: Left
+AlignOperands: true
+AlignTrailingComments: false
+AllowAllParametersOfDeclarationOnNextLine: false
+AllowShortBlocksOnASingleLine: false
+AllowShortCaseLabelsOnASingleLine: false
+AllowShortFunctionsOnASingleLine: None
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+AlwaysBreakAfterDefinitionReturnType: None
+AlwaysBreakAfterReturnType: None
+AlwaysBreakBeforeMultilineStrings: false
+AlwaysBreakTemplateDeclarations: false
+BinPackArguments: true
+BinPackParameters: true
+BraceWrapping:
+  AfterClass: false
+  AfterControlStatement: false
+  AfterEnum: false
+  AfterFunction: true
+  AfterNamespace: true
+  AfterObjCDeclaration: false
+  AfterStruct: false
+  AfterUnion: false
+  AfterExternBlock: false
+  BeforeCatch: false
+  BeforeElse: false
+  IndentBraces: false
+  SplitEmptyFunction: true
+  SplitEmptyRecord: true
+  SplitEmptyNamespace: true
+BreakBeforeBinaryOperators: None
+BreakBeforeBraces: Custom
+BreakBeforeInheritanceComma: false
+BreakBeforeTernaryOperators: false
+BreakConstructorInitializersBeforeComma: false
+BreakConstructorInitializers: BeforeComma
+BreakAfterJavaFieldAnnotations: false
+BreakStringLiterals: false
+ColumnLimit: 80
+CommentPragmas: '^ IWYU pragma:'
+CompactNamespaces: false
+ConstructorInitializerAllOnOneLineOrOnePerLine: false
+ConstructorInitializerIndentWidth: 8
+ContinuationIndentWidth: 8
+Cpp11BracedListStyle: false
+DerivePointerAlignment: false
+DisableFormat: false
+ExperimentalAutoDetectBinPacking: false
+FixNamespaceComments: false
+
+# Taken from:
+#   git grep -h '^#define [^[:space:]]*for_each[^[:space:]]*(' \
+#   | sed "s,^#define \([^[:space:]]*for_each[^[:space:]]*\)(.*$,  - '\1'," \
+#   | LC_ALL=C sort -u
+ForEachMacros:
+  - 'cil_list_for_each'
+  - 'cil_stack_for_each'
+  - 'cil_stack_for_each_starting_at'
+  - 'ebitmap_for_each_bit'
+  - 'ebitmap_for_each_positive_bit'
+
+IncludeBlocks: Preserve
+IncludeCategories:
+  - Regex: '.*'
+    Priority: 1
+IncludeIsMainRegex: '(Test)?$'
+IndentCaseLabels: false
+IndentGotoLabels: false
+IndentPPDirectives: None
+IndentWidth: 8
+IndentWrappedFunctionNames: false
+JavaScriptQuotes: Leave
+JavaScriptWrapImports: true
+KeepEmptyLinesAtTheStartOfBlocks: false
+MacroBlockBegin: ''
+MacroBlockEnd: ''
+MaxEmptyLinesToKeep: 1
+NamespaceIndentation: None
+ObjCBinPackProtocolList: Auto
+ObjCBlockIndentWidth: 8
+ObjCSpaceAfterProperty: true
+ObjCSpaceBeforeProtocolList: true
+
+# Taken from git's rules
+PenaltyBreakAssignment: 10
+PenaltyBreakBeforeFirstCallParameter: 30
+PenaltyBreakComment: 10
+PenaltyBreakFirstLessLess: 0
+PenaltyBreakString: 10
+PenaltyExcessCharacter: 100
+PenaltyReturnTypeOnItsOwnLine: 60
+
+PointerAlignment: Right
+ReflowComments: false
+SortIncludes: false
+SortUsingDeclarations: false
+SpaceAfterCStyleCast: false
+SpaceAfterTemplateKeyword: true
+SpaceBeforeAssignmentOperators: true
+SpaceBeforeCtorInitializerColon: true
+SpaceBeforeInheritanceColon: true
+SpaceBeforeParens: ControlStatementsExceptForEachMacros
+SpaceBeforeRangeBasedForLoopColon: true
+SpaceInEmptyParentheses: false
+SpacesBeforeTrailingComments: 1
+SpacesInAngles: false
+SpacesInContainerLiterals: false
+SpacesInCStyleCastParentheses: false
+SpacesInParentheses: false
+SpacesInSquareBrackets: false
+Standard: Cpp03
+TabWidth: 8
+UseTab: Always
+...
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index c501cf84..7ec8cb0f 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -53,6 +53,8 @@ When preparing patches, please follow these guidelines:
 -   Separate large patches into logical patches
 -   Patch descriptions must end with your "Signed-off-by" line. This means your
     code meets the Developer's certificate of origin, see below.
+-   C code should be formatted using clang-format, using the .clang-format
+    configuration file at the root of this repository.
 
 When adding new, large features or tools it is best to discuss the
 design on the mailing list prior to submitting the patch.
-- 
2.50.1