linux-kernel - [PATCH v6 28/57] drm_print: refine drm_debug

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Message-Id: <20220904214134.408619-29-jim.cromie@gmail.com>
Date:   Sun,  4 Sep 2022 15:41:05 -0600
From:   Jim Cromie <jim.cromie@...il.com>
To:     jbaron@...mai.com, gregkh@...uxfoundation.org,
        dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc:     daniel.vetter@...ll.ch, seanpaul@...omium.org, robdclark@...il.com,
        linux@...musvillemoes.dk, joe@...ches.com,
        Jim Cromie <jim.cromie@...il.com>
Subject: [PATCH v6 28/57] drm_print: refine drm_debug_enabled for jump-label

In order to use dynamic-debug's jump-label optimization in drm-debug,
its clarifying to refine drm_debug_enabled into 3 uses:

1.   drm_debug_enabled - legacy, public
2. __drm_debug_enabled - optimized for dyndbg jump-label enablement.
3.  _drm_debug_enabled - pr_debug instrumented, observable

1. The legacy version always checks the bits.

2. is privileged, for use by __drm_dbg(), __drm_dev_dbg(), which do an
early return unless the category is enabled.  For dyndbg builds, debug
callsites are selectively "pre-enabled", so __drm_debug_enabled()
short-circuits to true there.  Remaining callers of 1 may be able to
use 2, case by case.

3. is 1st wrapped in a macro, with a pr_debug, which reports each
usage in /proc/dynamic_debug/control, making it observable in the
logs.  The macro lets the pr_debug see the real caller, not an inline
function.

When plugged into 1, 3 identified ~10 remaining callers of the
function, leading to the follow-on cleanup patch, and would allow
activating the pr_debugs, estimating the callrate, and the potential
savings by using the wrapper macro.  It is unused ATM, but it fills
out the picture.

Signed-off-by: Jim Cromie <jim.cromie@...il.com>
---
 drivers/gpu/drm/drm_print.c |  4 ++--
 include/drm/drm_print.h     | 28 ++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
index 29a29949ad0b..cb203d63b286 100644
--- a/drivers/gpu/drm/drm_print.c
+++ b/drivers/gpu/drm/drm_print.c
@@ -285,7 +285,7 @@ void __drm_dev_dbg(const struct device *dev, enum drm_debug_category category,
 	struct va_format vaf;
 	va_list args;
 
-	if (!drm_debug_enabled(category))
+	if (!__drm_debug_enabled(category))
 		return;
 
 	va_start(args, format);
@@ -308,7 +308,7 @@ void ___drm_dbg(enum drm_debug_category category, const char *format, ...)
 	struct va_format vaf;
 	va_list args;
 
-	if (!drm_debug_enabled(category))
+	if (!__drm_debug_enabled(category))
 		return;
 
 	va_start(args, format);
diff --git a/include/drm/drm_print.h b/include/drm/drm_print.h
index dfdd81c3287c..7631b5fb669e 100644
--- a/include/drm/drm_print.h
+++ b/include/drm/drm_print.h
@@ -321,11 +321,39 @@ enum drm_debug_category {
 	DRM_UT_DRMRES
 };
 
+/*
+ * 3 name flavors of drm_debug_enabled:
+ *   drm_debug_enabled - public/legacy, always checks bits
+ *  _drm_debug_enabled - instrumented to observe call-rates, est overheads.
+ * __drm_debug_enabled - privileged - knows jump-label state, can short-circuit
+ */
 static inline bool drm_debug_enabled(enum drm_debug_category category)
 {
 	return unlikely(__drm_debug & BIT(category));
 }
 
+/*
+ * Wrap fn in macro, so that the pr_debug sees the actual caller, not
+ * the inline fn.  Using this name creates a callsite entry / control
+ * point in /proc/dynamic_debug/control.
+ */
+#define _drm_debug_enabled(category)				\
+	({							\
+		pr_debug("todo: maybe avoid via dyndbg\n");     \
+		drm_debug_enabled(category);			\
+	})
+
+#if defined(CONFIG_DRM_USE_DYNAMIC_DEBUG)
+/*
+ * dyndbg is wrapping the drm.debug API, so as to avoid the runtime
+ * bit-test overheads of drm_debug_enabled() in those api calls.
+ * In this case, executed callsites are known enabled, so true.
+ */
+#define __drm_debug_enabled(category)	true
+#else
+#define __drm_debug_enabled(category)	drm_debug_enabled(category)
+#endif
+
 /*
  * struct device based logging
  *
-- 
2.37.2