docs: coresight: Document snapshot mode
Because of the resctriction imposed by the internal memory buffer
of some CoreSight sink, using the framework in snapshot mode can
be a little trickly. As such document the process to make user
experience more enjoyable.
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
diff --git a/Documentation/trace/coresight.txt b/Documentation/trace/coresight.txt
index efbc832..6c602ae 100644
--- a/Documentation/trace/coresight.txt
+++ b/Documentation/trace/coresight.txt
@@ -363,7 +363,6 @@
Note that only 64-bit programs are currently supported - further work is
required to support instruction decode of 32-bit Arm programs.
-
Generating coverage files for Feedback Directed Optimization: AutoFDO
---------------------------------------------------------------------
@@ -394,6 +393,46 @@
Bubble sorting array of 30000 elements
5806 ms
+2.2) Snapshot mode:
+
+Using perf's built-in snapshot mode with CoreSight tracers is supported - to
+do so the '-S' command line option needs to be specified. Since current sink
+devices are used in double-buffer mode when operated from the perf interface,
+the size of the perf ring buffer needs to be adjusted to match the size of the
+buffer used by the CoreSight sinks. From the perf command line it is possible
+to specify the number of pages to use for a session using the '-m,X' option,
+where X is the amount of pages.
+
+The system memory buffer used by ETR devices is automatically adjusted
+to match the size of the perf ring buffer and as such does not need to be
+modified on the perf command line. For ETB and ETF devices the perf ring
+buffer size need to be adjusted to match the size of the internal buffer.
+
+The following examples assume a system page size of 4096 byte:
+
+ # cat /sys/bus/coresight/devices/20010000.etb/mgmt/rdp
+ 0x2000
+ # perf record -e cs_etm/@20010000.etf/ -S -m,8 --per-thread $APP
+
+ # cat /sys/bus/coresight/devices/20010000.etf/buffer_size
+ 0x10000
+ # perf record -e cs_etm/@20010000.etf/ -S -m,16 --per-thread $APP
+
+ # perf record -e cs_etm/@20070000.etr/ -S --per-thread $APP
+
+Once an application is launched trace snapshot are collected by sending the
+USR2 message to the process being monitored:
+
+ # perf record -e cs_etm/@20070000.etr/ -S --per-thread $APP &
+ [1] 14808
+ # kill -USR2 14808
+ ...
+ ...
+ # kill -USR2 14808
+ ...
+ ...
+ # kill 14808
+
How to use the STM module
-------------------------