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

[Louis Chauvet]

Le 02/04/2025 à 19:41, Jim Cromie a écrit :
> 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 at vger.kernel.org
> Signed-off-by: Jim Cromie <jim.cromie at 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

Can you also clarify this in the Controlling dynamic debug Behaviour? At 
least something like "Some printk may not be affected by >control 
because of classmaps, see Dynamic Debug classmaps for more information".

Or maybe integrate this section just after/inside Controlling dynamic 
debug Behaviour. So a user is not surprised if his command does not have 
the expected effect.

> +
> +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.

Very clear, thanks!

> +
> +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.

-- 
Louis Chauvet, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com