drm/i915: add register macro definition style guide

This is not to try to force a new style; this is my interpretation of
what the most common existing style is.

With hopes I don't need to answer so many questions about style going
forward.

Start a new style section in the i915 document to bolt the register
style guide into.

v2: vertical alignment, incorporate to kernel-doc, and more

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9de4a5b1bea4e76461c70a1dd66751581de0124f.1502368010.git.jani.nikula@intel.com

Documentation/gpu/i915.rst

@@ -480,3 +480,17 @@ specific details than found in the more high-level sections.
 
 .. kernel-doc:: drivers/gpu/drm/i915/i915_perf.c
    :internal:
+
+Style
+=====
+
+The drm/i915 driver codebase has some style rules in addition to (and, in some
+cases, deviating from) the kernel coding style.
+
+Register macro definition style
+-------------------------------
+
+The style guide for ``i915_reg.h``.
+
+.. kernel-doc:: drivers/gpu/drm/i915/i915_reg.h
+   :doc: The i915 register macro definition style guide

drivers/gpu/drm/i915/i915_reg.h

@@ -25,6 +25,97 @@
 #ifndef _I915_REG_H_
 #define _I915_REG_H_
 
+/**
+ * DOC: The i915 register macro definition style guide
+ *
+ * Follow the style described here for new macros, and while changing existing
+ * macros. Do **not** mass change existing definitions just to update the style.
+ *
+ * Layout
+ * ''''''
+ *
+ * Keep helper macros near the top. For example, _PIPE() and friends.
+ *
+ * Prefix macros that generally should not be used outside of this file with
+ * underscore '_'. For example, _PIPE() and friends, single instances of
+ * registers that are defined solely for the use by function-like macros.
+ *
+ * Avoid using the underscore prefixed macros outside of this file. There are
+ * exceptions, but keep them to a minimum.
+ *
+ * There are two basic types of register definitions: Single registers and
+ * register groups. Register groups are registers which have two or more
+ * instances, for example one per pipe, port, transcoder, etc. Register groups
+ * should be defined using function-like macros.
+ *
+ * For single registers, define the register offset first, followed by register
+ * contents.
+ *
+ * For register groups, define the register instance offsets first, prefixed
+ * with underscore, followed by a function-like macro choosing the right
+ * instance based on the parameter, followed by register contents.
+ *
+ * Define the register contents (i.e. bit and bit field macros) from most
+ * significant to least significant bit. Indent the register content macros
+ * using two extra spaces between ``#define`` and the macro name.
+ *
+ * For bit fields, define a ``_MASK`` and a ``_SHIFT`` macro. Define bit field
+ * contents so that they are already shifted in place, and can be directly
+ * OR'd. For convenience, function-like macros may be used to define bit fields,
+ * but do note that the macros may be needed to read as well as write the
+ * register contents.
+ *
+ * Define bits using ``(1 << N)`` instead of ``BIT(N)``. We may change this in
+ * the future, but this is the prevailing style. Do **not** add ``_BIT`` suffix
+ * to the name.
+ *
+ * Group the register and its contents together without blank lines, separate
+ * from other registers and their contents with one blank line.
+ *
+ * Indent macro values from macro names using TABs. Align values vertically. Use
+ * braces in macro values as needed to avoid unintended precedence after macro
+ * substitution. Use spaces in macro values according to kernel coding
+ * style. Use lower case in hexadecimal values.
+ *
+ * Naming
+ * ''''''
+ *
+ * Try to name registers according to the specs. If the register name changes in
+ * the specs from platform to another, stick to the original name.
+ *
+ * Try to re-use existing register macro definitions. Only add new macros for
+ * new register offsets, or when the register contents have changed enough to
+ * warrant a full redefinition.
+ *
+ * When a register macro changes for a new platform, prefix the new macro using
+ * the platform acronym or generation. For example, ``SKL_`` or ``GEN8_``. The
+ * prefix signifies the start platform/generation using the register.
+ *
+ * When a bit (field) macro changes or gets added for a new platform, while
+ * retaining the existing register macro, add a platform acronym or generation
+ * suffix to the name. For example, ``_SKL`` or ``_GEN8``.
+ *
+ * Examples
+ * ''''''''
+ *
+ * (Note that the values in the example are indented using spaces instead of
+ * TABs to avoid misalignment in generated documentation. Use TABs in the
+ * definitions.)::
+ *
+ *  #define _FOO_A                      0xf000
+ *  #define _FOO_B                      0xf001
+ *  #define FOO(pipe)                   _MMIO_PIPE(pipe, _FOO_A, _FOO_B)
+ *  #define   FOO_ENABLE                (1 << 31)
+ *  #define   FOO_MODE_MASK             (0xf << 16)
+ *  #define   FOO_MODE_SHIFT            16
+ *  #define   FOO_MODE_BAR              (0 << 16)
+ *  #define   FOO_MODE_BAZ              (1 << 16)
+ *  #define   FOO_MODE_QUX_SNB          (2 << 16)
+ *
+ *  #define BAR                         _MMIO(0xb000)
+ *  #define GEN8_BAR                    _MMIO(0xb888)
+ */
+
 typedef struct {
 	uint32_t reg;
 } i915_reg_t;