[PATCH] dyndbg: add special aux_print framework

[Jim Cromie]

Sean Paul seanpaul at chromium.org proposed, in
https://patchwork.freedesktop.org/series/78133/
drm/trace: Mirror DRM debug logs to tracefs

The problem with the approach is that its built upon splitting
drm_debug_enabled() into syslog & trace flavors, which clashes rather
profoundly with the strategy of obsoleting it using dyndbg.

Instead, this puts the print-to-trace decision after the is-it-enabled
test (which is a noop), so it has near zero additional cost.

This is preliminary, Proof-of-Concept, and about 2 hrs old.
But its surprisingly simple:

 - add a new struct _ddebug member: (*aux_print)(char *format, ...)
 - add a new S/special flag to check !!aux_print
 - if S, invoke the function to handle the prepared vaf

It intrinsically allows any number of alternate printf-ish consumers,
but only 1 active per callsite.  I have another patchset that
eliminates some of the data redundancies like this, it can be
extended.

It may also prove to be a generic way to implement the netdev & ibdev
variants of __dynamic_pr_debug.

It just needs a mechanism to set the per-callsite pointer to a
printf-ish function to consume the pr_debug output, a tighter/better
function prototype, and a wrapper on drm_trace_printf to bundle up the
args and comport with the prototype, which can evolve to suit this 1st
client.

it is on top of:
https://patchwork.freedesktop.org/series/92544/
(v4 on lkml, v2 in patchwork)

Signed-off-by: Jim Cromie <jim.cromie at gmail.com>
---
 include/linux/dynamic_debug.h |  7 ++++++-
 lib/dynamic_debug.c           | 22 +++++++++++++++++++---
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index 677ad176b167..0d068e8ed7aa 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -22,6 +22,7 @@ struct _ddebug {
 	const char *function;
 	const char *filename;
 	const char *format;
+	int (*aux_print)(char *, void *, void *);
 	unsigned int lineno:18;
 	/*
 	 * The flags field controls the behaviour at the callsite.
@@ -29,7 +30,11 @@ struct _ddebug {
 	 * writes commands to <debugfs>/dynamic_debug/control
 	 */
 #define _DPRINTK_FLAGS_NONE	0
-#define _DPRINTK_FLAGS_PRINT	(1<<0) /* printk() a message using the format */
+#define _DPRINTK_FLAGS_PRINT		(1<<0) /* printk() a message */
+#define _DPRINTK_FLAGS_PRINT_AUX	(1<<5) /* call (*aux_print) */
+
+#define _DPRINTK_ENABLED (_DPRINTK_FLAGS_PRINT | _DPRINTK_FLAGS_PRINT_AUX)
+
 #define _DPRINTK_FLAGS_INCL_MODNAME	(1<<1)
 #define _DPRINTK_FLAGS_INCL_FUNCNAME	(1<<2)
 #define _DPRINTK_FLAGS_INCL_LINENO	(1<<3)
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 045e1cf92c44..7bbdedabe6f1 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -85,6 +85,7 @@ static inline const char *trim_prefix(const char *path)
 
 static struct { unsigned flag:8; char opt_char; } opt_array[] = {
 	{ _DPRINTK_FLAGS_PRINT, 'p' },
+	{ _DPRINTK_FLAGS_PRINT_AUX, 'S' },
 	{ _DPRINTK_FLAGS_INCL_MODNAME, 'm' },
 	{ _DPRINTK_FLAGS_INCL_FUNCNAME, 'f' },
 	{ _DPRINTK_FLAGS_INCL_LINENO, 'l' },
@@ -206,10 +207,10 @@ static int ddebug_change(const struct ddebug_query *query,
 			if (newflags == dp->flags)
 				continue;
 #ifdef CONFIG_JUMP_LABEL
-			if (dp->flags & _DPRINTK_FLAGS_PRINT) {
-				if (!(modifiers->flags & _DPRINTK_FLAGS_PRINT))
+			if (dp->flags & _DPRINTK_ENABLED) {
+				if (!(modifiers->flags & _DPRINTK_ENABLED))
 					static_branch_disable(&dp->key.dd_key_true);
-			} else if (modifiers->flags & _DPRINTK_FLAGS_PRINT)
+			} else if (modifiers->flags & _DPRINTK_ENABLED)
 				static_branch_enable(&dp->key.dd_key_true);
 #endif
 			dp->flags = newflags;
@@ -639,6 +640,21 @@ void __dynamic_pr_debug(struct _ddebug *descriptor, const char *fmt, ...)
 
 	printk(KERN_DEBUG "%s%pV", dynamic_emit_prefix(descriptor, buf), &vaf);
 
+	if (descriptor->flags & _DPRINTK_FLAGS_PRINT_AUX) {
+		/* our model:
+		drm_trace_printf("%s%s[" DRM_NAME ":%ps] %pV",
+				 dev ? dev_name(dev) : "", dev ? " " : "",
+				 __builtin_return_address(0), &vaf);
+		*/
+		pr_info("reached check aux\n");
+
+		if (descriptor->aux_channel) {
+			pr_info("calling aux\n");
+			(*descriptor->aux_channel)
+				("%s[DRM_mumble :%ps] %pV", buf,
+				 __builtin_return_address(0), &vaf);
+		}
+	}
 	va_end(args);
 }
 EXPORT_SYMBOL(__dynamic_pr_debug);
-- 
2.31.1