[PATCH] doc-rst: ipu3: clarification on data type conversion of IEFD CU

[PATCH] doc-rst: ipu3: clarification on data type conversion of IEFD CU

[bingbu.cao]

From: Bingbu Cao <bingbu.cao@intel.com>

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: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
---
 drivers/staging/media/ipu3/TODO                 | 2 --
 drivers/staging/media/ipu3/include/intel-ipu3.h | 5 +++++
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/media/ipu3/TODO b/drivers/staging/media/ipu3/TODO
index 5e55baeaea1a..f2542a7a09b5 100644
--- a/drivers/staging/media/ipu3/TODO
+++ b/drivers/staging/media/ipu3/TODO
@@ -13,8 +13,6 @@ 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.
diff --git a/drivers/staging/media/ipu3/include/intel-ipu3.h b/drivers/staging/media/ipu3/include/intel-ipu3.h
index c7cd27efac8a..08eaa0bad0de 100644
--- a/drivers/staging/media/ipu3/include/intel-ipu3.h
+++ b/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.
  */
-- 
2.7.4