perf docs: Add description for stderr

'perf report' displays recorded data on the screen and emits warnings
and debug messages in the status line (last one on screen).

perf also supports the possibility to write all debug messages to stderr
(instead of writing them to the status line).

This is achieved with the following command:

  # ./perf --debug stderr=1 report -vvvvv -i ~/fast.data 2>/tmp/2
  # ll /tmp/2
  -rw-rw-r-- 1 tmricht tmricht 5420835 May  7 13:46 /tmp/2
  #

The usage of variable stderr=1 is not documented, so add it to the perf
man page.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Link: http://lkml.kernel.org/r/20190513080220.91966-1-tmricht@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/tools/perf/Documentation/perf.txt b/tools/perf/Documentation/perf.txt
index 864e37597252..401f0ed67439 100644
--- a/tools/perf/Documentation/perf.txt
+++ b/tools/perf/Documentation/perf.txt
@@ -22,6 +22,8 @@ OPTIONS
           verbose          - general debug messages
           ordered-events   - ordered events object debug messages
           data-convert     - data convert command debug messages
+          stderr           - write debug output (option -v) to stderr
+                             in browser mode
 
 --buildid-dir::
         Setup buildid cache directory. It has higher priority than