Documentation: fpga: dfl: add description for performance reporting support

This patch adds description for performance reporting support for
Device Feature List (DFL) based FPGA.

Signed-off-by: Xu Yilun <[e-mail]>
Signed-off-by: Wu Hao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/fpga/dfl.rst

···
      management information (current temperature, thresholds, threshold status,
      etc.).
 
+ Performance reporting
+     performance counters are exposed through perf PMU APIs. Standard perf tool
+     can be used to monitor all available perf events. Please see performance
+     counter section below for more detailed information.
+
 
 FIU - PORT
 ==========
···
 
 	/sys/class/fpga_region/<regionX>/<dfl-fme.n>/dev
 	/sys/class/fpga_region/<regionX>/<dfl-port.n>/dev
+
+
+Performance Counters
+====================
+Performance reporting is one private feature implemented in FME. It could
+supports several independent, system-wide, device counter sets in hardware to
+monitor and count for performance events, including "basic", "cache", "fabric",
+"vtd" and "vtd_sip" counters. Users could use standard perf tool to monitor
+FPGA cache hit/miss rate, transaction number, interface clock counter of AFU
+and other FPGA performance events.
+
+Different FPGA devices may have different counter sets, depending on hardware
+implementation. E.g., some discrete FPGA cards don't have any cache. User could
+use "perf list" to check which perf events are supported by target hardware.
+
+In order to allow user to use standard perf API to access these performance
+counters, driver creates a perf PMU, and related sysfs interfaces in
+/sys/bus/event_source/devices/dfl_fme* to describe available perf events and
+configuration options.
+
+The "format" directory describes the format of the config field of struct
+perf_event_attr. There are 3 bitfields for config: "evtype" defines which type
+the perf event belongs to; "event" is the identity of the event within its
+category; "portid" is introduced to decide counters set to monitor on FPGA
+overall data or a specific port.
+
+The "events" directory describes the configuration templates for all available
+events which can be used with perf tool directly. For example, fab_mmio_read
+has the configuration "event=0x06,evtype=0x02,portid=0xff", which shows this
+event belongs to fabric type (0x02), the local event id is 0x06 and it is for
+overall monitoring (portid=0xff).
+
+Example usage of perf::
+
+  $# perf list |grep dfl_fme
+
+  dfl_fme0/fab_mmio_read/                              [Kernel PMU event]
+  <...>
+  dfl_fme0/fab_port_mmio_read,portid=?/                [Kernel PMU event]
+  <...>
+
+  $# perf stat -a -e dfl_fme0/fab_mmio_read/ <command>
+  or
+  $# perf stat -a -e dfl_fme0/event=0x06,evtype=0x02,portid=0xff/ <command>
+  or
+  $# perf stat -a -e dfl_fme0/config=0xff2006/ <command>
+
+Another example, fab_port_mmio_read monitors mmio read of a specific port. So
+its configuration template is "event=0x06,evtype=0x01,portid=?". The portid
+should be explicitly set.
+
+Its usage of perf::
+
+  $# perf stat -a -e dfl_fme0/fab_port_mmio_read,portid=0x0/ <command>
+  or
+  $# perf stat -a -e dfl_fme0/event=0x06,evtype=0x02,portid=0x0/ <command>
+  or
+  $# perf stat -a -e dfl_fme0/config=0x2006/ <command>
+
+Please note for fabric counters, overall perf events (fab_*) and port perf
+events (fab_port_*) actually share one set of counters in hardware, so it can't
+monitor both at the same time. If this set of counters is configured to monitor
+overall data, then per port perf data is not supported. See below example::
+
+  $# perf stat -e dfl_fme0/fab_mmio_read/,dfl_fme0/fab_port_mmio_write,\
+                                                    portid=0/ sleep 1
+
+  Performance counter stats for 'system wide':
+
+                 3      dfl_fme0/fab_mmio_read/
+   <not supported>      dfl_fme0/fab_port_mmio_write,portid=0x0/
+
+       1.001750904 seconds time elapsed
+
+The driver also provides a "cpumask" sysfs attribute, which contains only one
+CPU id used to access these perf events. Counting on multiple CPU is not allowed
+since they are system-wide counters on FPGA device.
+
+The current driver does not support sampling. So "perf record" is unsupported.
 
 
 Add new FIUs support