[PATCH v3 05/22] tracefs: add __get_str_strip_nl - RFC

["Łukasz Bartosik" <lb@semihalf.com>]

From: Jim Cromie <jim.cromie@gmail.com>

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 [1] 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.

[1] https://lore.kernel.org/lkml/20200825153338.17061-1-vincent.whitchurch@axis.com

Cc: <rostedt@goodmis.org>
Cc: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: <daniel@ffwll.ch>
Cc: <pmladek@suse.com>
Cc: <sergey.senozhatsky@gmail.com>
Cc: <john.ogness@linutronix.de>
Cc: Simon Ser <contact@emersion.fr>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Łukasz Bartosik <lb@semihalf.com>
---
 include/trace/stages/stage3_trace_output.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/trace/stages/stage3_trace_output.h b/include/trace/stages/stage3_trace_output.h
index c1fb1355d309..283533a17e62 100644
--- a/include/trace/stages/stage3_trace_output.h
+++ b/include/trace/stages/stage3_trace_output.h
@@ -19,6 +19,19 @@
 #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 = trace_seq_buffer_ptr(p);                      \
+		size_t len;                                             \
+		trace_seq_printf(p, "%s", __get_str(field));            \
+		trace_seq_putc(p, '\0');                                \
+		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 + 					\
-- 
2.43.0.472.g3155946c3a-goog