media: ipu3.rst: add explanation for running mode

Add some explanation of the ImgU running mode and add more information
about firmware selection and running mode usage.
Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/ipu3.rst

@@ -231,12 +231,31 @@ Configuring the Intel IPU3
 The IPU3 ImgU pipelines can be configured using the Media Controller, defined at
 :ref:`media_controller`.
 
-Firmware binary selection
+Running mode and firmware binary selection
 -------------------------
+ImgU works based on firmware, currently the ImgU firmware support run 2 pipes in
+time-sharing with single input frame data. Each pipe can run at certain mode -
+"VIDEO" or "STILL", "VIDEO" mode is commonly used for video frames capture, and
+"STILL" is used for still frame capture. However, you can also select "VIDEO" to
+capture still frames if you want to capture images with less system load and
+power. For "STILL" mode, ImgU will try to use smaller BDS factor and output
+larger bayer frame for further YUV processing than "VIDEO" mode to get high
+quality images. Besides, "STILL" mode need XNR3 to do noise reduction, hence
+"STILL" mode will need more power and memory bandwidth than "VIDEO" mode. TNR
+will be enabled in "VIDEO" mode and bypassed by "STILL" mode. ImgU is running at
+“VIDEO” mode by default, the user can use v4l2 control V4L2_CID_INTEL_IPU3_MODE
+(currently defined in drivers/staging/media/ipu3/include/intel-ipu3.h) to query
+and set the running mode. For user, there is no difference for buffer queueing
+between the "VIDEO" and "STILL" mode, mandatory input and main output node
+should be enabled and buffers need be queued, the statistics and the view-finder
+queues are optional.
+
+The firmware binary will be selected according to current running mode, such log
+"using binary if_to_osys_striped " or "using binary if_to_osys_primary_striped"
+could be observed if you enable the ImgU dynamic debug, the binary
+if_to_osys_striped is selected for "VIDEO" and the binary
+"if_to_osys_primary_striped" is selected for "STILL".
 
-The firmware binary is selected using the V4L2_CID_INTEL_IPU3_MODE, currently
-defined in drivers/staging/media/ipu3/include/intel-ipu3.h . "VIDEO" and "STILL"
-modes are available.
 
 Processing the image in raw Bayer format
 ----------------------------------------

drivers/staging/media/ipu3/TODO

@@ -10,7 +10,3 @@ staging directory.
 
 - Elaborate the functionality of different selection rectangles in driver
   documentation. This may require driver changes as well.
-
-- Document different operation modes, and which buffer queues are relevant
-  in each mode. To process an image, which queues require a buffer an in
-  which ones is it optional?