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Date:   Sun, 31 Mar 2019 18:48:24 -0500
From:   Tom Zanussi <zanussi@...nel.org>
To:     rostedt@...dmis.org
Cc:     tglx@...utronix.de, mhiramat@...nel.org, namhyung@...nel.org,
        bigeasy@...utronix.de, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: [PATCH v5 10/11] tracing: Add tracing/error_log Documentation

From: Tom Zanussi <tom.zanussi@...ux.intel.com>

Move most of the hist trigger extended error documentation to
ftrace.rst and expand on it to fully document tracing/error_log.

Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>
Acked-by: Namhyung Kim <namhyung@...nel.org>
---
 Documentation/trace/ftrace.rst    | 31 +++++++++++++++++++++++++++++++
 Documentation/trace/histogram.rst | 16 ++--------------
 2 files changed, 33 insertions(+), 14 deletions(-)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 7c5e6d6ab5d1..809b39d066ee 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -765,6 +765,37 @@ Here is the list of current tracers that may be configured.
 	tracers from tracing simply echo "nop" into
 	current_tracer.
 
+Error conditions
+----------------
+
+  For most ftrace commands, failure modes are obvious and communicated
+  using standard return codes.
+
+  For other more involved commands, extended error information may be
+  available via the tracing/error_log file.  For the commands that
+  support it, reading the tracing/error_log file after an error will
+  display more detailed information about what went wrong, if
+  information is available.  The tracing/error_log file is a circular
+  error log displaying a small number (currently, 8) of ftrace errors
+  for the last (8) failed commands.
+
+  The extended error information and usage takes the form shown in
+  this example::
+
+    # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
+    echo: write error: Invalid argument
+
+    # cat /sys/kernel/debug/tracing/error_log
+    [ 5348.887237] location: error: Couldn't yyy: zzz
+      Command: xxx
+               ^
+    [ 7517.023364] location: error: Bad rrr: sss
+      Command: ppp qqq
+                   ^
+
+  To clear the error log, echo the empty string into it::
+
+    # echo > /sys/kernel/debug/tracing/error_log
 
 Examples of using the tracer
 ----------------------------
diff --git a/Documentation/trace/histogram.rst b/Documentation/trace/histogram.rst
index 0ea59d45aef1..7612c7ad5715 100644
--- a/Documentation/trace/histogram.rst
+++ b/Documentation/trace/histogram.rst
@@ -199,20 +199,8 @@ Extended error information
 
   For some error conditions encountered when invoking a hist trigger
   command, extended error information is available via the
-  corresponding event's 'hist' file.  Reading the hist file after an
-  error will display more detailed information about what went wrong,
-  if information is available.  This extended error information will
-  be available until the next hist trigger command for that event.
-
-  If available for a given error condition, the extended error
-  information and usage takes the following form::
-
-    # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
-    echo: write error: Invalid argument
-
-    # cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/hist
-    ERROR: Couldn't yyy: zzz
-      Last command: xxx
+  tracing/error_log file.  See Error Conditions in
+  :file:`Documentation/trace/ftrace.rst` for details.
 
 6.2 'hist' trigger examples
 ---------------------------
-- 
2.14.1

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