[PATCH RFC v2] media: open.rst: document devnode-centric and mc-centric types

[Mauro Carvalho Chehab <mchehab@osg.samsung.com>]

From: "mchehab@s-opensource.com" <mchehab@s-opensource.com>

When we added support for omap3, back in 2010, we added a new
type of V4L2 devices that aren't fully controlled via the V4L2
device node. Yet, we never made it clear, at the V4L2 spec,
about the differences between both types.

Let's document them with the current implementation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
---
 Documentation/media/uapi/v4l/open.rst | 47 +++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/Documentation/media/uapi/v4l/open.rst b/Documentation/media/uapi/v4l/open.rst
index afd116edb40d..cf522d9bb53c 100644
--- a/Documentation/media/uapi/v4l/open.rst
+++ b/Documentation/media/uapi/v4l/open.rst
@@ -6,6 +6,53 @@
 Opening and Closing Devices
 ***************************
 
+Types of V4L2 device control
+============================
+
+V4L2 devices are usually complex: they're implemented via a main driver and
+often several additional drivers. The main driver always exposes one or
+more **V4L2 device** devnodes (see :ref:`v4l2_device_naming`). The other
+devices are called **V4L2 sub-devices**. They are usually controlled via a
+serial bus (I2C or SMBus).
+
+When V4L2 started, there was only one type of device control. The entire
+device was controlled via the **V4L2 device nodes**. We refer to this
+kind of control as **V4L2 device-centric** (or, simply, **device-centric**).
+
+Since the end of 2010, a new type of V4L2 device control was added in order
+to support complex devices that are common on embedded systems. Those
+devices are controlled mainly via the media controller and sub-devices.
+So, they're called: **media controller centric** (or, simply,
+"**mc-centric**").
+
+On **device-centric** control, the device and their corresponding hardware
+pipelines are controlled via the **V4L2 device** node. They may optionally
+expose the hardware pipelines via the
+:ref:`media controller API <media_controller>`.
+
+On a **mc-centric**, before using the V4L2 device, it is required to
+set the hardware pipelines via the
+:ref:`media controller API <media_controller>`. On those devices, the
+sub-devices' configuration can be controlled via the
+:ref:`sub-device API <subdev>`, with creates one device node per sub device.
+
+In summary, on **mc-centric** devices:
+
+- The **V4L2 device** node is mainly responsible for controlling the
+  streaming features;
+- The **media controller device** is responsible to setup the pipelines
+  and image settings (like size and format);
+- The **V4L2 sub-devices** are responsible for sub-device
+  specific settings.
+
+.. note::
+
+   It is forbidden for **device-centric** devices to expose V4L2
+   sub-devices via :ref:`sub-device API <subdev>`, although this
+   might change in the future.
+
+
+.. _v4l2_device_naming:
 
 Device Naming
 =============
-- 
2.13.5