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-rw-r--r--Documentation/trace/intel_th.rst28
1 files changed, 27 insertions, 1 deletions
diff --git a/Documentation/trace/intel_th.rst b/Documentation/trace/intel_th.rst
index baa12eb09ef4..70b7126eaeeb 100644
--- a/Documentation/trace/intel_th.rst
+++ b/Documentation/trace/intel_th.rst
@@ -44,7 +44,8 @@ Documentation/trace/stm.rst for more information on that.
MSU can be configured to collect trace data into a system memory
buffer, which can later on be read from its device nodes via read() or
-mmap() interface.
+mmap() interface and directed to a "software sink" driver that will
+consume the data and/or relay it further.
On the whole, Intel(R) Trace Hub does not require any special
userspace software to function; everything can be configured, started
@@ -122,3 +123,28 @@ In order to enable the host mode, set the 'host_mode' parameter of the
will show up on the intel_th bus. Also, trace configuration and
capture controlling attribute groups of the 'gth' device will not be
exposed. The 'sth' device will operate as usual.
+
+Software Sinks
+--------------
+
+The Memory Storage Unit (MSU) driver provides an in-kernel API for
+drivers to register themselves as software sinks for the trace data.
+Such drivers can further export the data via other devices, such as
+USB device controllers or network cards.
+
+The API has two main parts::
+ - notifying the software sink that a particular window is full, and
+ "locking" that window, that is, making it unavailable for the trace
+ collection; when this happens, the MSU driver will automatically
+ switch to the next window in the buffer if it is unlocked, or stop
+ the trace capture if it's not;
+ - tracking the "locked" state of windows and providing a way for the
+ software sink driver to notify the MSU driver when a window is
+ unlocked and can be used again to collect trace data.
+
+An example sink driver, msu-sink illustrates the implementation of a
+software sink. Functionally, it simply unlocks windows as soon as they
+are full, keeping the MSU running in a circular buffer mode. Unlike the
+"multi" mode, it will fill out all the windows in the buffer as opposed
+to just the first one. It can be enabled by writing "sink" to the "mode"
+file (assuming msu-sink.ko is loaded).