media: media api: Try to make enum usage clearer

This clarifies which side of the calls is responsible for doing what to
which parts of the struct. It also expands the terse description of the
access algorithm into more prose-like, active voice description, which
trades conciseness for ease of comprehension.

Fixed: typo "format" -> "frame size" in enum-frame-size
Added: no holes in the enumeration
Added: enumerations per what?
Added: who fills in what in calls
Changed: "given" -> "specified"

[Sakari Ailus: Rewrap text]
Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@puri.sm>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Documentation/userspace-api/media/v4l/vidioc-subdev-enum-frame-size.rst

@@ -31,18 +31,30 @@ Arguments
 Description
 ===========
 
-This ioctl allows applications to enumerate all frame sizes supported by
-a sub-device on the given pad for the given media bus format. Supported
-formats can be retrieved with the
+This ioctl allows applications to access the enumeration of frame sizes
+supported by a sub-device on the specified pad
+for the specified media bus format.
+Supported formats can be retrieved with the
 :ref:`VIDIOC_SUBDEV_ENUM_MBUS_CODE`
 ioctl.
 
-To enumerate frame sizes applications initialize the ``pad``, ``which``
-, ``code`` and ``index`` fields of the struct
-:c:type:`v4l2_subdev_mbus_code_enum` and
-call the :ref:`VIDIOC_SUBDEV_ENUM_FRAME_SIZE` ioctl with a pointer to the
-structure. Drivers fill the minimum and maximum frame sizes or return an
-EINVAL error code if one of the input parameters is invalid.
+The enumerations are defined by the driver, and indexed using the ``index`` field
+of the struct :c:type:`v4l2_subdev_mbus_code_enum`.
+Each pair of ``pad`` and ``code`` correspond to a separate enumeration.
+Each enumeration starts with the ``index`` of 0, and
+the lowest invalid index marks the end of the enumeration.
+
+Therefore, to enumerate frame sizes allowed on the specified pad
+and using the specified mbus format, initialize the
+``pad``, ``which``, and ``code`` fields to desired values,
+and set ``index`` to 0.
+Then call the :ref:`VIDIOC_SUBDEV_ENUM_FRAME_SIZE` ioctl with a pointer to the
+structure.
+
+A successful call will return with minimum and maximum frame sizes filled in.
+Repeat with increasing ``index`` until ``EINVAL`` is received.
+``EINVAL`` means that either no more entries are available in the enumeration,
+or that an input parameter was invalid.
 
 Sub-devices that only support discrete frame sizes (such as most
 sensors) will return one or more frame sizes with identical minimum and
@@ -72,26 +84,28 @@ information about try formats.
 
     * - __u32
       - ``index``
-      - Number of the format in the enumeration, set by the application.
+      - Index of the frame size in the enumeration belonging to the given pad
+	and format. Filled in by the application.
     * - __u32
       - ``pad``
       - Pad number as reported by the media controller API.
+	Filled in by the application.
     * - __u32
       - ``code``
       - The media bus format code, as defined in
-	:ref:`v4l2-mbus-format`.
+	:ref:`v4l2-mbus-format`. Filled in by the application.
     * - __u32
       - ``min_width``
-      - Minimum frame width, in pixels.
+      - Minimum frame width, in pixels. Filled in by the driver.
     * - __u32
       - ``max_width``
-      - Maximum frame width, in pixels.
+      - Maximum frame width, in pixels. Filled in by the driver.
     * - __u32
       - ``min_height``
-      - Minimum frame height, in pixels.
+      - Minimum frame height, in pixels. Filled in by the driver.
     * - __u32
       - ``max_height``
-      - Maximum frame height, in pixels.
+      - Maximum frame height, in pixels. Filled in by the driver.
     * - __u32
       - ``which``
       - Frame sizes to be enumerated, from enum