[PATCH v2 05/15] tracefs: add __get_str_strip_nl - RFC

[Łukasz Bartosik]

From: Jim Cromie <jim.cromie@xxxxxxxxx>

This variant of __get_str() removes the trailing newline. It is for
use by printk/debug-ish events which already have a trailing newline.
It is here to support:

https://lore.kernel.org/lkml/
20200825153338.17061-1-vincent.whitchurch@xxxxxxxx/
which taught dyndbg to send pr_debug() msgs to tracefs, via -x/T flag.

It "reused" the include/trace/events/printk.h console event,
which does the following:

       TP_fast_assign(
               /*
                * Each trace entry is printed in a new line.
                * If the msg finishes with '\n', cut it off
                * to avoid blank lines in the trace.
                */
               if (len > 0 && (msg[len-1] == '\n'))
                       len -= 1;

               memcpy(__get_str(s), msg, len);
               __get_str(s)[len] = 0;
	),

That trim work could be avoided, *if* all pr_debug() callers are
known to have no '\n' to strip.  While that's not true for *all*
callsites, it is 99+% true for DRM.debug callsites, and can be made
true for some subsets of prdbg/dyndbg callsites.

WANTED: macros to validate that a literal format-str has or doesn't
have a trailing newline, or to provide or trim trailing newline(s?).
Should be usable in TP_printk* defns, for use in new event defns.

Cc: <rostedt@xxxxxxxxxxx>
Cc: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
Cc: <daniel@xxxxxxxx>
Cc: <pmladek@xxxxxxxx>
Cc: <sergey.senozhatsky@xxxxxxxxx>
Cc: <john.ogness@xxxxxxxxxxxxx>
Cc: Simon Ser <contact@xxxxxxxxxxx>
Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
Signed-off-by: Jim Cromie <jim.cromie@xxxxxxxxx>
Signed-off-by: Łukasz Bartosik <lb@xxxxxxxxxxxx>
---
 include/trace/stages/stage3_trace_output.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/trace/stages/stage3_trace_output.h b/include/trace/stages/stage3_trace_output.h
index c1fb1355d309..92a79bd5c0cd 100644
--- a/include/trace/stages/stage3_trace_output.h
+++ b/include/trace/stages/stage3_trace_output.h
@@ -19,6 +19,15 @@
 #undef __get_str
 #define __get_str(field) ((char *)__get_dynamic_array(field))
 
+#undef __get_str_strip_nl
+#define __get_str_strip_nl(field)					\
+	({								\
+		char *s = __get_str(field);				\
+		size_t len = strlen(s);					\
+		if (len && s[len-1] == '\n')				\
+			s[len-1] = '\0'; s;				\
+	})
+
 #undef __get_rel_dynamic_array
 #define __get_rel_dynamic_array(field)					\
 		((void *)__entry + 					\
-- 
2.43.0.rc2.451.g8631bc7472-goog