[RFC PATCH v2 4/4] docs: perf: Minimal introduction the the CXL PMU device and driver.

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

Very basic introduction to the device and the current driver support
provided. I expect to expand on this in future versions of this patch
set.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

--
RFC:
- I'll post separately about this shortly, but it seems very odd
  to me that there is no way to assign a parent to an event_sources
  device.  As a result we get the messy approach of playing match
  the name to figure out what the CPMU instance is connected to.
---
 Documentation/admin-guide/perf/cxl.rst   | 60 ++++++++++++++++++++++++
 Documentation/admin-guide/perf/index.rst |  1 +
 2 files changed, 61 insertions(+)

diff --git a/Documentation/admin-guide/perf/cxl.rst b/Documentation/admin-guide/perf/cxl.rst
new file mode 100644
index 000000000000..6ffc057591fb
--- /dev/null
+++ b/Documentation/admin-guide/perf/cxl.rst
@@ -0,0 +1,60 @@
+======================================
+CXL Performance Monitoring Unit (CPMU)
+======================================
+
+The CXL rev 3.0 specification provides a definition of CXL Performance
+Monitoring Unit in section 13.2: Performance Monitoring.
+
+CXL components (e.g. Root Port, Switch Upstream Port, End Point) may have
+any number of CPMU instances. CPMU capabilities are fully discoverable from
+the devices. The specification provides event definitions for all CXL protocol
+message types and a set of additional events for things commonly counted on
+CXL devices (e.g. DRAM events).
+
+CPMU driver
+===========
+
+The CPMU driver register a perf PMU with the name cpmu<id> on the CXL bus.
+
+    /sys/bus/cxl/device/cpmu<id>
+
+The associated PMU is registered as
+
+   /sys/bus/event_sources/devices/cpmu<id>
+
+In common with other CXL bus devices, the id has no specific meaning and the
+relationship to specific CXL device should be established via the device parent
+of the device on the CXL bus.
+
+PMU driver provides description of available events and filter options in sysfs.
+
+The "format" directory describes all formats of the config (event vendor id,
+group id and mask) config1 (threshold, filter enables) and config2 (filter
+parameters) fields of the perf_event_attr structure.  The "events" directory
+describes all documented events show in perf list.
+
+The events shown in perf list are the most fine grained events with a single
+bit of the event mask set. More general events may be enable by setting
+multiple mask bits in config. For example, all Device to Host Read Requests
+may be captured on a single counter by setting the bits for all of
+
+* d2h_req_rdcurr
+* d2h_req_rdown
+* d2h_req_rdshared
+* d2h_req_rdany
+* d2h_req_rdownnodata
+
+Example of usage::
+
+  $#perf list
+  cpmu0/clock_ticks/                                 [Kernel PMU event]
+  cpmu0/d2h_req_itomwr/                              [Kernel PMU event]
+  cpmu0/d2h_req_rdany/                               [Kernel PMU event]
+  cpmu0/d2h_req_rdcurr/                              [Kernel PMU event]
+  -----------------------------------------------------------
+
+  $# perf stat -e cpmu0/clock_ticks/ -e cpmu0/d2h_req_itowrm/
+
+The driver does not support sampling. So "perf record" and attaching to
+a task are unsupported.
+
diff --git a/Documentation/admin-guide/perf/index.rst b/Documentation/admin-guide/perf/index.rst
index 9c9ece88ce53..f12cfe76e1f0 100644
--- a/Documentation/admin-guide/perf/index.rst
+++ b/Documentation/admin-guide/perf/index.rst
@@ -18,3 +18,4 @@ Performance monitor support
    xgene-pmu
    arm_dsu_pmu
    thunderx2-pmu
+   cxl
-- 
2.32.0