media: doc-rst: ipu3: clarification on data type conversion of IEFD CU

The data type conversion of the IEFD CU inputs in ipu3 uapi
is ambiguities, add some clarification to help user to
understand this conversion.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Suggested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/staging/media/ipu3/TODO

@@ -12,8 +12,6 @@ staging directory.
   Comments on configuring v4l2 subdevs for CIO2 and ImgU.
 
 - uAPI documentation:
-  Further clarification on some ambiguities such as data type conversion of
-  IEFD CU inputs. (Sakari)
   Move acronyms to doc-rst file. (Mauro)
 
 - Switch to yavta from v4l2n in driver docs.

drivers/staging/media/ipu3/include/intel-ipu3.h

@@ -1217,6 +1217,11 @@ struct ipu3_uapi_shd_config {
  *
  * All CU inputs are unsigned, they will be converted to signed when written
  * to register, i.e. a01 will be written to 9 bit register in s4.4 format.
+ * The data precision s4.4 means 4 bits for integer parts and 4 bits for the
+ * fractional part, the first bit indicates positive or negative value.
+ * For userspace software (commonly the imaging library), the computation for
+ * the CU slope values should be based on the slope resolution 1/16 (binary
+ * 0.0001 - the minimal interval value), the slope value range is [-256, +255].
  * This applies to &ipu3_uapi_iefd_cux6_ed, &ipu3_uapi_iefd_cux2_1,
  * &ipu3_uapi_iefd_cux2_1, &ipu3_uapi_iefd_cux4 and &ipu3_uapi_iefd_cux6_rad.
  */