[PATCH v6 10/28] media: Documentation: v4l: Document source routes

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Document how internal pads are used on source routes. Use the IMX219
camera sensor as an example.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
---
 .../userspace-api/media/v4l/dev-subdev.rst    | 179 ++++++++++++++++++
 1 file changed, 179 insertions(+)

diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
index a387e8a15b8d..fb73a95401c3 100644
--- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
+++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
@@ -553,6 +553,27 @@ A stream at a specific point in the media pipeline is identified by the
 sub-device and a (pad, stream) pair. For sub-devices that do not support
 multiplexed streams the 'stream' field is always 0.
 
+.. _v4l2-subdev-source-routes:
+
+Internal pads and source routes
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Cases where a single sub-device source pad is traversed by multiple streams one
+or more of which originate from within the sub-device itself are special as
+there is no external sink pad for such routes. In those cases, the sources of
+the internally generated streams are represented by internal sink pads, which
+are sink pads that have the :ref:`MEDIA_PAD_FL_INTERNAL <MEDIA-PAD-FL-INTERNAL>`
+pad flag set.
+
+Internal pads have all the properties of an external pad, including formats and
+selections. The format in this case is the source format of the stream. An
+internal pad always has a single stream only (0).
+
+*Source routes* are routes from an internal sink pad to an external source
+pad. In most cases source routes are not modifiable but they can be activated
+and deactivated using the :ref:`V4L2_SUBDEV_ROUTE_FL_ACTIVE
+<v4l2-subdev-routing-flags>` flag, depending on driver capabilities.
+
 Interaction between routes, streams, formats and selections
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -668,3 +689,161 @@ To configure this pipeline, the userspace must take the following steps:
    the configurations along the stream towards the receiver, using
    :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls to configure each
    stream endpoint in each sub-device.
+
+Internal pads setup example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A simple example of a multiplexed stream setup might be as follows:
+
+- A CCS camera sensor source sub-device, with one sink pad (0), one source pad
+  (1), an internal sink pad (2) that represents the source of embedded
+  data. There are two routes, one from the sink pad to the source, and another
+  from the internal sink pad to the source pad. The embedded data stream needs
+  to be enabled by activating the related route. The configuration of the rest
+  of the CCS sub-devices is omitted from this example.
+
+- Multiplexer bridge (Bridge). The bridge has one sink pad, connected to the
+  sensor (pad 0), and one source pad (pad 1), which outputs two streams.
+
+- Receiver in the SoC (Receiver). The receiver has a single sink pad (pad 0),
+  connected to the bridge, and two source pads (pads 1-2), going to the DMA
+  engine. The receiver demultiplexes the incoming streams to the source pads.
+
+- DMA Engines in the SoC (DMA Engine), one for each stream. Each DMA engine is
+  connected to a single source pad in the receiver.
+
+The sensor, the bridge and the receiver are modeled as V4L2 sub-devices,
+exposed to userspace via /dev/v4l-subdevX device nodes. The DMA engines are
+modeled as V4L2 devices, exposed to userspace via /dev/videoX nodes.
+
+To configure this pipeline, the userspace must take the following steps:
+
+1) Set up media links between entities: connect the sensors to the bridge,
+   bridge to the receiver, and the receiver to the DMA engines. This step does
+   not differ from normal non-multiplexed media controller setup.
+
+2) Configure routing
+
+.. flat-table:: Camera sensor
+    :header-rows: 1
+
+    * - Sink Pad/Stream
+      - Source Pad/Stream
+      - Routing Flags
+      - Comments
+    * - 0/0
+      - 1/0
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Pixel data stream from the sink pad
+    * - 2/0
+      - 1/1
+      - **V4L2_SUBDEV_ROUTE_FL_ACTIVE**
+      - Metadata stream from the internal sink pad
+
+.. flat-table:: Bridge routing table
+    :header-rows: 1
+
+    * - Sink Pad/Stream
+      - Source Pad/Stream
+      - Routing Flags
+      - Comments
+    * - 0/0
+      - 1/0
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Pixel data stream from camera sensor
+    * - 0/1
+      - 1/1
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Metadata stream from camera sensor
+
+.. flat-table:: Receiver routing table
+    :header-rows:  1
+
+    * - Sink Pad/Stream
+      - Source Pad/Stream
+      - Routing Flags
+      - Comments
+    * - 0/0
+      - 1/0
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Pixel data stream from camera sensor
+    * - 0/1
+      - 2/0
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Metadata stream from camera sensor
+
+The options available in sensor's routing configuration are dictated by
+hardware capabilities: typically camera sensors always produce image data
+stream while the embedded data stream typically can be either enabled or
+disabled.
+
+3) Configure formats and selections
+
+   This example assumes that the formats are propagated from sink pad to the
+   source pad as-is. The tables contain fields of both struct v4l2_subdev_format
+   and struct v4l2_mbus_framefmt.
+
+.. flat-table:: Formats set on the sub-devices. Bold values are set, others are
+                static or propagated.
+    :header-rows: 1
+    :fill-cells:
+
+    * - Sub-device
+      - Pad/Stream
+      - Width
+      - Height
+      - Code
+    * - :rspan:`3` Camera sensor sub-device (IMX219)
+      - 1/0
+      - 3296
+      - 2480
+      - MEDIA_BUS_FMT_SRGGB10
+    * - 0/0
+      - **3296**
+      - **2480**
+      - **MEDIA_BUS_FMT_SRGGB10**
+    * - 2/0
+      - 3296
+      - 2
+      - MEDIA_BUS_FMT_IMX219_EMBEDDED
+    * - 1/1
+      - 3296
+      - 2
+      - MEDIA_BUS_FMT_META_10
+    * - :rspan:`3` Bridge
+      - 0/0
+      - **3296**
+      - **2480**
+      - **MEDIA_BUS_FMT_SRGGB10**
+    * - 1/0
+      - 3296
+      - 2480
+      - MEDIA_BUS_FMT_SRGGB10
+    * - 0/1
+      - **3296**
+      - **2**
+      - **MEDIA_BUS_FMT_META_10**
+    * - 1/1
+      - 3296
+      - 2
+      - MEDIA_BUS_FMT_META_10
+    * - :rspan:`3` Receiver
+      - 0/0
+      - **3296**
+      - **2480**
+      - **MEDIA_BUS_FMT_SRGGB10**
+    * - 1/0
+      - 3296
+      - 2480
+      - MEDIA_BUS_FMT_SRGGB10
+    * - 0/1
+      - **3296**
+      - **2**
+      - **MEDIA_BUS_FMT_META_10**
+    * - 2/0
+      - 3296
+      - 2
+      - MEDIA_BUS_FMT_META_10
+
+The embedded data format does not need to be configured as the format is
+dictated by the pixel data format in this case.
-- 
2.39.2