media: doc: media/v4l-drivers: Add Qualcomm Camera Subsystem driver document

Add a document to describe Qualcomm Camera Subsystem driver.
Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/v4l-drivers/qcom_camss.rst

+.. include:: <isonum.txt>
+
+Qualcomm Camera Subsystem driver
+================================
+
+Introduction
+------------
+
+This file documents the Qualcomm Camera Subsystem driver located under
+drivers/media/platform/qcom/camss-8x16.
+
+The current version of the driver supports the Camera Subsystem found on
+Qualcomm MSM8916 and APQ8016 processors.
+
+The driver implements V4L2, Media controller and V4L2 subdev interfaces.
+Camera sensor using V4L2 subdev interface in the kernel is supported.
+
+The driver is implemented using as a reference the Qualcomm Camera Subsystem
+driver for Android as found in Code Aurora [#f1]_.
+
+
+Qualcomm Camera Subsystem hardware
+----------------------------------
+
+The Camera Subsystem hardware found on 8x16 processors and supported by the
+driver consists of:
+
+- 2 CSIPHY modules. They handle the Physical layer of the CSI2 receivers.
+  A separate camera sensor can be connected to each of the CSIPHY module;
+- 2 CSID (CSI Decoder) modules. They handle the Protocol and Application layer
+  of the CSI2 receivers. A CSID can decode data stream from any of the CSIPHY.
+  Each CSID also contains a TG (Test Generator) block which can generate
+  artificial input data for test purposes;
+- ISPIF (ISP Interface) module. Handles the routing of the data streams from
+  the CSIDs to the inputs of the VFE;
+- VFE (Video Front End) module. Contains a pipeline of image processing hardware
+  blocks. The VFE has different input interfaces. The PIX input interface feeds
+  the input data to the image processing pipeline. Three RDI input interfaces
+  bypass the image processing pipeline. The VFE also contains the AXI bus
+  interface which writes the output data to memory.
+
+
+Supported functionality
+-----------------------
+
+The current version of the driver supports:
+
+- input from camera sensor via CSIPHY;
+- generation of test input data by the TG in CSID;
+- raw dump of the input data to memory. RDI interface of VFE is supported.
+  PIX interface (ISP processing, statistics engines, resize/crop, format
+  conversion) is not supported in the current version;
+- concurrent and independent usage of two data inputs - could be camera sensors
+  and/or TG.
+
+
+Driver Architecture and Design
+------------------------------
+
+The driver implements the V4L2 subdev interface. With the goal to model the
+hardware links between the modules and to expose a clean, logical and usable
+interface, the driver is split into V4L2 sub-devices as follows:
+
+- 2 CSIPHY sub-devices - each CSIPHY is represented by a single sub-device;
+- 2 CSID sub-devices - each CSID is represented by a single sub-device;
+- 2 ISPIF sub-devices - ISPIF is represented by a number of sub-devices equal
+  to the number of CSID sub-devices;
+- 3 VFE sub-devices - VFE is represented by a number of sub-devices equal to
+  the number of RDI input interfaces.
+
+The considerations to split the driver in this particular way are as follows:
+
+- representing CSIPHY and CSID modules by a separate sub-device for each module
+  allows to model the hardware links between these modules;
+- representing VFE by a separate sub-devices for each RDI input interface allows
+  to use the three RDI interfaces concurently and independently as this is
+  supported by the hardware;
+- representing ISPIF by a number of sub-devices equal to the number of CSID
+  sub-devices allows to create linear media controller pipelines when using two
+  cameras simultaneously. This avoids branches in the pipelines which otherwise
+  will require a) userspace and b) media framework (e.g. power on/off
+  operations) to  make assumptions about the data flow from a sink pad to a
+  source pad on a single media entity.
+
+Each VFE sub-device is linked to a separate video device node.
+
+The complete list of the media entities (V4L2 sub-devices and video device
+nodes) is as follows:
+
+- msm_csiphy0
+- msm_csiphy1
+- msm_csid0
+- msm_csid1
+- msm_ispif0
+- msm_ispif1
+- msm_vfe0_rdi0
+- msm_vfe0_video0
+- msm_vfe0_rdi1
+- msm_vfe0_video1
+- msm_vfe0_rdi2
+- msm_vfe0_video2
+
+
+Implementation
+--------------
+
+Runtime configuration of the hardware (updating settings while streaming) is
+not required to implement the currently supported functionality. The complete
+configuration on each hardware module is applied on STREAMON ioctl based on
+the current active media links, formats and controls set.
+
+
+Documentation
+-------------
+
+APQ8016 Specification:
+https://developer.qualcomm.com/download/sd410/snapdragon-410-processor-device-specification.pdf
+Referenced 2016-11-24.
+
+
+References
+----------
+
+.. [#f1] https://source.codeaurora.org/quic/la/kernel/msm-3.10/