[PATCH v3 29/54] docs/dyndbg: add classmap info to howto

[Jim Cromie <jim.cromie@gmail.com>]

Describe the 3 API macros providing dynamic_debug's classmaps

DYNDBG_CLASSMAP_DEFINE - create & export a classmap
DYNDBG_CLASSMAP_USE    - refer to exported map
DYNDBG_CLASSMAP_PARAM  - bind control param to the classmap
DYNDBG_CLASSMAP_PARAM_REF + use module's storage - __drm_debug

TBD: some of this might be over-specification, or just over-talked.

NB: The _DEFINE & _USE model makes the user dependent on the definer,
just like EXPORT_SYMBOL(__drm_debug) already does.

cc: linux-doc@vger.kernel.org
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
v3- rework protection around PARAM

v0.5 adjustments per Randy Dunlap
v0.7 checkpatch fixes
v0.8 more
v0.9 rewords

fixup-howto
---
 .../admin-guide/dynamic-debug-howto.rst       | 93 +++++++++++++++++++
 1 file changed, 93 insertions(+)

diff --git a/Documentation/admin-guide/dynamic-debug-howto.rst b/Documentation/admin-guide/dynamic-debug-howto.rst
index 1ceadf4f28f9..5eb4ae3b2f27 100644
--- a/Documentation/admin-guide/dynamic-debug-howto.rst
+++ b/Documentation/admin-guide/dynamic-debug-howto.rst
@@ -394,3 +394,96 @@ just a shortcut for ``print_hex_dump(KERN_DEBUG)``.
 For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
 its ``prefix_str`` argument, if it is constant string; or ``hexdump``
 in case ``prefix_str`` is built dynamically.
+
+Dynamic Debug classmaps
+=======================
+
+The "class" keyword selects prdbgs based on author supplied,
+domain-oriented names.  This complements the nested-scope keywords:
+module, file, function, line.
+
+The main difference from the others: classes must be named to be
+changed.  This protects them from generic overwrite:
+
+  # IOW this cannot undo any DRM.debug settings
+  :#> ddcmd -p
+
+This protection is needed in order to honor the ABI, settings done
+there must be respected:
+
+  :#> echo 0x1ff > /sys/module/drm/parameters/debug
+
+So each class must be enabled individually (no wildcards):
+
+  :#> ddcmd class DRM_UT_CORE +p
+  :#> ddcmd class DRM_UT_KMS +p
+  # or more selectively
+  :#> ddcmd class DRM_UT_CORE module drm +p
+
+That makes direct >control wordy and annoying, but it is a secondary
+interface; it is not intended to replace the ABI, just slide in
+underneath and reimplement it.
+
+However, since the param is the ABI, if a classmap DEFINEr doesn't
+also add a _CLASSMAP_PARAM, there is no ABI, and no protection is
+needed.  In that case, class'd prdbgs would be enabled/disabled by
+legacy (class-less) queries.
+
+
+Dynamic Debug Classmap API
+==========================
+
+DRM.debug is built upon:
+  ABI in /sys/module/drm/parameters/debug
+      the bits set all DRM_UT_* together
+  ~23 categorized api macros: drm_dbg_<T>()
+      all calling drm_{,dev}dbg(DRM_UT_*, ....)
+  ~5000 calls to the api macros across drivers/gpu/drm/*
+
+The const short ints are good for optimizing compilers; a primary
+classmaps design goal was to preserve those opporunities for
+optimization.  So basically .classid === category.
+
+Then we use the drm_categories DRM_UT_* enum for both the classnames
+(stringified enum symbols) and their numeric values.
+
+Its expected that future users will also use an enum-defined
+categorization scheme like DRM's, and dyndbg can be adapted under them
+similarly.
+
+DYNAMIC_DEBUG_CLASSMAP_DEFINE(var,type,_base,classnames) - this maps
+classnames (a list of strings) onto class-ids consecutively, starting
+at _base, it also maps the names onto CLASSMAP_PARAM bits 0..N.
+
+DYNAMIC_DEBUG_CLASSMAP_USE(var) - modules call this to refer to the
+var _DEFINEd elsewhere (and exported).
+
+Classmaps are opt-in: modules invoke _DEFINE or _USE to authorize
+dyndbg to update those classes.  "class FOO" queries are validated
+against the classes, this finds the classid to alter; classes are not
+directly selectable by their classid.
+
+There are 2 types of classmaps:
+
+ DD_CLASS_TYPE_DISJOINT_BITS: classes are independent, like DRM.debug
+ DD_CLASS_TYPE_LEVEL_NUM: classes are relative, ordered (V3 > V2)
+
+DYNAMIC_DEBUG_CLASSMAP_PARAM - modelled after module_param_cb, it
+refers to a DEFINEd classmap, and associates it to the param's
+data-store.  This state is then applied to DEFINEr and USEr modules
+when they're modprobed.
+
+The PARAM interface also enforces the DD_CLASS_TYPE_LEVEL_NUM relation
+amongst the contained classnames; all classes are independent in the
+control parser itself; there is no implied meaning in names like "V4".
+
+Modules or module-groups (drm & drivers) can define multiple
+classmaps, as long as they (all the classmaps) share the limited 0..62
+per-module-group _class_id range, without overlap.  If a module
+encounters a conflict between 2 classmaps its USEing, we can extend
+the _USE macro with an offset to allow avoiding the conflicting range.
+
+``#define DEBUG`` will enable all pr_debugs in scope, including any
+class'd ones.  This won't be reflected in the PARAM readback value,
+but the class'd pr_debug callsites can be forced off by toggling the
+classmap-kparam all-on then all-off.
-- 
2.49.0