[PATCH v2 20/23] dynamic_debug: introduce CONFIG_DYNAMIC_DEBUG_RELATIVE_POINTERS

[Rasmus Villemoes]

Based on the same idea for struct bug_entry, an architecture can opt-in
to use relative pointers in struct _ddebug. It only makes sense for 64
bit architectures, where one saves 16 bytes per entry (out of 40 or 56,
depending on CONFIG_JUMP_LABEL). The architecture is responsible for
providing a suitable DEFINE_DYNAMIC_DEBUG_METADATA macro in
<asm/dynamic_debug.h>.

Acked-by: Jason Baron <jbaron@xxxxxxxxxx>
Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
 include/linux/dynamic_debug.h | 14 ++++++++++++++
 lib/Kconfig.debug             |  3 +++
 lib/dynamic_debug.c           | 20 ++++++++++++++++++++
 3 files changed, 37 insertions(+)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index e1be30e8422b..cc4222a20aa1 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -16,10 +16,17 @@ struct _ddebug {
 	 * These fields are used to drive the user interface
 	 * for selecting and displaying debug callsites.
 	 */
+#ifdef CONFIG_DYNAMIC_DEBUG_RELATIVE_POINTERS
+	signed int modname_disp;
+	signed int function_disp;
+	signed int filename_disp;
+	signed int format_disp;
+#else
 	const char *modname;
 	const char *function;
 	const char *filename;
 	const char *format;
+#endif
 	/*
 	 * The flags field controls the behaviour at the callsite.
 	 * The bits here are changed dynamically when the user
@@ -70,6 +77,12 @@ void __dynamic_netdev_dbg(struct _ddebug *descriptor,
 			  const struct net_device *dev,
 			  const char *fmt, ...);
 
+#ifdef CONFIG_DYNAMIC_DEBUG_RELATIVE_POINTERS
+#include <asm/dynamic_debug.h>
+#ifndef DEFINE_DYNAMIC_DEBUG_METADATA
+# error "asm/dynamic_debug.h must provide definition of DEFINE_DYNAMIC_DEBUG_METADATA"
+#endif
+#else
 #define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt)		\
 	static struct _ddebug  __aligned(8)			\
 	__attribute__((section("__verbose"))) name = {		\
@@ -80,6 +93,7 @@ void __dynamic_netdev_dbg(struct _ddebug *descriptor,
 		.flags_lineno = _DPRINTK_FLAGS_LINENO_INIT,	\
 		_DPRINTK_KEY_INIT				\
 	}
+#endif
 
 #ifdef HAVE_JUMP_LABEL
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 4966c4fbe7f7..a4113f746826 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -146,6 +146,9 @@ config DYNAMIC_DEBUG
 	  See Documentation/admin-guide/dynamic-debug-howto.rst for additional
 	  information.
 
+config DYNAMIC_DEBUG_RELATIVE_POINTERS
+	bool
+
 endmenu # "printk and dmesg options"
 
 menu "Compile-time checks and compiler options"
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 9d4c840ff0de..61e61b36e479 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -37,6 +37,24 @@
 #include <linux/device.h>
 #include <linux/netdevice.h>
 
+#ifdef CONFIG_DYNAMIC_DEBUG_RELATIVE_POINTERS
+static inline const char *dd_modname(const struct _ddebug *dd)
+{
+	return (const char *)dd + dd->modname_disp;
+}
+static inline const char *dd_function(const struct _ddebug *dd)
+{
+	return (const char *)dd + dd->function_disp;
+}
+static inline const char *dd_filename(const struct _ddebug *dd)
+{
+	return (const char *)dd + dd->filename_disp;
+}
+static inline const char *dd_format(const struct _ddebug *dd)
+{
+	return (const char *)dd + dd->format_disp;
+}
+#else
 static inline const char *dd_modname(const struct _ddebug *dd)
 {
 	return dd->modname;
@@ -53,6 +71,8 @@ static inline const char *dd_format(const struct _ddebug *dd)
 {
 	return dd->format;
 }
+#endif
+
 static inline unsigned dd_lineno(const struct _ddebug *dd)
 {
 	return dd->flags_lineno >> 8;
-- 
2.19.1.3.g1d92a00e68