libcamera: Add "Software ISP benchmarking" documentation

Add a "Software ISP benchmarking" documentation section which describes
the performance/power consumption measurements used during
the Software ISP's development.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

Documentation/index.rst

@@ -24,3 +24,4 @@
    Lens driver requirements <lens_driver_requirements>
    Python Bindings <python-bindings>
    Camera Sensor Model <camera-sensor-model>
+   SoftwareISP Benchmarking <software-isp-benchmarking>

Documentation/meson.build

@@ -80,6 +80,7 @@ if sphinx.found()
         'lens_driver_requirements.rst',
         'python-bindings.rst',
         'sensor_driver_requirements.rst',
+        'software-isp-benchmarking.rst',
        '../README.rst',
     ]
 

Documentation/software-isp-benchmarking.rst

@@ -0,0 +1,82 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. _software-isp-benchmarking:
+
+Software ISP benchmarking
+=========================
+
+The Software ISP is paricular sensitive to performance regressions
+therefor it is a good idea to always benchmark the Software ISP
+before and after making changes to it and ensure that there are
+no performance regressions.
+
+DebayerCpu class builtin benchmark
+----------------------------------
+
+The DebayerCpu class has a builtin benchmark. This benchmark
+measures the time spend on processing (collecting statistics
+and debayering) only, it does not measure the time spend on
+capturing or outputting the frames.
+
+The builtin benchmark always runs. So this can be used by simply
+running "cam" or "qcam" with a pipeline using the Software ISP.
+
+When it runs it will skip measuring the first 30 frames to
+allow the caches and the CPU temperature (turbo-ing) to warm-up
+and then it measures 30 fps and shows the total and per frame
+processing time using an info level log message:
+
+.. code-block::
+
+   INFO Debayer debayer_cpu.cpp:907 Processed 30 frames in 244317us, 8143 us/frame
+
+To get stable measurements it is advised to disable any other processes which
+may cause significant CPU usage (e.g. disable wifi, bluetooth and browsers).
+When possible it is also advisable to disable CPU turbo-ing and
+frequency-scaling.
+
+For example when benchmarking on a Lenovo ThinkPad X1 Yoga Gen 8, with
+the charger plugged in, the CPU can be fixed to run at 2 GHz using:
+
+.. code-block::
+
+   sudo x86_energy_perf_policy --turbo-enable 0
+   sudo cpupower frequency-set -d 2GHz -u 2GHz
+
+with these settings the builtin bench reports a processing time of ~7.8ms/frame
+on this laptop for FHD SGRBG10 (unpacked) bayer data.
+
+Measuring power consumption
+---------------------------
+
+Since the Software ISP is often used on mobile devices it is also
+important to measure power consumption and ensure that that does
+not regress.
+
+For example to measure power consumption on a Lenovo ThinkPad X1 Yoga Gen 8
+it needs to be running on battery and it should be configured with its
+platform-profile (/sys/firmware/acpi/platform_profile) set to balanced and
+with its default turbo and frequency-scaling behavior to match real world usage.
+
+Then start qcam to capture a FHD picture at 30 fps and position the qcam window
+so that it is fully visible. After this run the following command to monitor
+the power consumption:
+
+.. code-block::
+
+   watch -n 10 cat /sys/class/power_supply/BAT0/power_now /sys/class/hwmon/hwmon6/fan?_input
+
+Note this not only measures the power consumption in ųW it also monitors
+the speed of this laptop's 2 fans. This is important because depending on
+the ambient temperature the 2 fans may spin up while testing and this
+will cause an additional power consumption of approx. 0.5W messing up
+the measurement.
+
+After starting qcam + the watch command let the laptop sit without using
+it for 2 minutes for the readings to stabilize. Then check that the fans
+have not turned on and manually take a couple of consecutive power readings
+and avarage these.
+
+On the example Lenovo ThinkPad X1 Yoga Gen 8 laptop this results in
+a measured power consumption of approx. 13W while running qcam versus
+approx. 4-5W while setting idle with its OLED panel on.