[PATCH v6 52/57] dyndbg: add DEFINE_DYNAMIC_DEBUG_TABLE, use it tacitly RFC

[Jim Cromie]

This defines a new macro (tbd: tweak name) which is a special version
of DEFINE_DYNAMIC_DEBUG_METADATA_CLS().  Unlike that macro, which
declares callsites in pairs of desc/site recs.

This macro declares a pair of header records, which the linker script
then places once (.gnu.linkonce.) into the front of the respective
sections.  For builtins, the .linkonce gives us a single header
preceding all ~3-5k callsites.

It uses __unused, __weak, .gnu.linkonce to avoid trouble with multiple
inclusions, and should not consume space when not linked to by a
callsite in a loadable module.

Atypically, this macro is also tacitly invoked in the header, which is
indirectly included by printk.h.  This means 2 things: it mostly
insures that source files will declare it at least once, and that
multiple decls resolve to the identical storage address.

I did this with the expectation that I could let the linker compute
the offset and initialize ._index accordingly, but that resulted in
some "linker cant compute" error.  Ive initialized it in
__ddebug_add_module() instead.

Signed-off-by: Jim Cromie <jim.cromie@xxxxxxxxx>
---
 include/linux/dynamic_debug.h | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index f23608c38a79..23d3d2882882 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -220,6 +220,32 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
 #define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt)		\
 	DEFINE_DYNAMIC_DEBUG_METADATA_CLS(name, _DPRINTK_CLASS_DFLT, fmt)
 
+/*
+ * DEFINE_DYNAMIC_DEBUG_TABLE(): This is a special version of
+ * DEFINE_DYNAMIC_DEBUG_METADATA().  It creates a single pair of
+ * header records, which the linker scripts place in front of their
+ * respective sections.
+ *
+ * To allow for a robust runtime test in dynamic_debug_init(), the
+ * pair of records is hardwired into a self-reference loop which can
+ * be validated before the field is altered to support _ddebug_map_site()
+ */
+#define DEFINE_DYNAMIC_DEBUG_TABLE()					\
+	/* forward decl, allowing loopback pointer */			\
+	struct _ddebug_hdr __used __aligned(8)				\
+		__section(".gnu.linkonce.dyndbg")			\
+		_LINKONCE_dyndbg_header;				\
+	struct _ddebug_site_hdr __used __aligned(8)			\
+		__section(".gnu.linkonce.dyndbg_site")			\
+		_LINKONCE_dyndbg_site_header = {			\
+		._uplink = (void *) &_LINKONCE_dyndbg_header		\
+	};								\
+	struct _ddebug_hdr __used __aligned(8)				\
+		__section(".gnu.linkonce.dyndbg")			\
+		_LINKONCE_dyndbg_header = {				\
+		._uplink = (void *) &_LINKONCE_dyndbg_site_header	\
+	}
+
 #ifdef CONFIG_JUMP_LABEL
 
 #ifdef DEBUG
@@ -390,4 +416,12 @@ static inline int param_get_dyndbg_classes(char *buffer, const struct kernel_par
 
 extern const struct kernel_param_ops param_ops_dyndbg_classes;
 
+#if ((defined(CONFIG_DYNAMIC_DEBUG) ||					\
+      (defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))) \
+     && defined(KBUILD_MODNAME) && !defined(NO_DYNAMIC_DEBUG_TABLE))
+
+/* transparently invoked, except when -DNO_DYNAMIC_DEBUG_TABLE */
+DEFINE_DYNAMIC_DEBUG_TABLE();
+#endif
+
 #endif
-- 
2.37.2