Change up config lookup and add documentation for it

README.md

@@ -14,3 +14,37 @@ $ sudo ninja install
 # Developing
 
 See the mailing list and issue tracker on https://sr.ht/~martijnbraam/Megapixels/
+
+# Config
+
+Megapixels checks multiple locations for it's configuration file and uses the first one it finds.
+As first step it will get the first compatible name in the device tree, in the case of a PinePhone
+this might be "pine64,pinephone-1.2". Then that dtname will be used as the filename in the search
+path in this order:
+
+* $XDG_CONFIG_DIR/megapixels/config/$dtname.ini
+* ~/.config/megapixels/config/$dtname.ini
+* /etc/megapixels/config/$dtname.ini
+* /usr/share/megapixels/config/$dtname.ini
+
+The files in /usr/share/megapixels should be the config files distributed in this repository. The other
+locations allow the user or distribution to override config.
+
+## Config file format
+
+Configuration files are INI format files. 
+
+### [device]
+
+This provides global info, currently only the `csi` key exists, telling megapixels which device in the 
+media-ctl tree is the interface to the kernel. This should provide the /dev/video* node.
+
+### [rear] and [front]
+
+These are the sections describing the sensors.
+
+* `driver=ov5640` the name of the media node that provides the sensor and it's /dev/v4l-subdev* node.
+* `width=640` and `height=480` the resolution to use for the sensor
+* `rate=15` the refresh rate in fps to use for the sensor
+* `fmt=BGGR8` sets the pixel and bus formats used when capturing from the sensor, only BGGR8 is fully supported
+* `rotate=90` the rotation angle to make the sensor match the screen