[media] s5p-fimc: Add driver documentation

Samsung S5P/EXYNOS4 FIMC driver

Copyright (C) 2012 Samsung Electronics Co., Ltd.

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The FIMC (Fully Interactive Mobile Camera) device available in Samsung

SoC Application Processors is an integrated camera host interface, color

space converter, image resizer and rotator.  It's also capable of capturing

data from LCD controller (FIMD) through the SoC internal writeback data

path.  There are multiple FIMC instances in the SoCs (up to 4), having

slightly different capabilities, like pixel alignment constraints, rotator

availability, LCD writeback support, etc. The driver is located at

drivers/media/video/s5p-fimc directory.

1. Supported SoCs

=================

S5PC100 (mem-to-mem only), S5PV210, EXYNOS4210

2. Supported features

=====================

 - camera parallel interface capture (ITU-R.BT601/565);

 - camera serial interface capture (MIPI-CSI2);

 - memory-to-memory processing (color space conversion, scaling, mirror

   and rotation);

 - dynamic pipeline re-configuration at runtime (re-attachment of any FIMC

   instance to any parallel video input or any MIPI-CSI front-end);

 - runtime PM and system wide suspend/resume

Not currently supported:

 - LCD writeback input

 - per frame clock gating (mem-to-mem)

3. Files partitioning

=====================

- media device driver

  drivers/media/video/s5p-fimc/fimc-mdevice.[ch]

 - camera capture video device driver

  drivers/media/video/s5p-fimc/fimc-capture.c

 - MIPI-CSI2 receiver subdev

  drivers/media/video/s5p-fimc/mipi-csis.[ch]

 - video post-processor (mem-to-mem)

  drivers/media/video/s5p-fimc/fimc-core.c

 - common files

  drivers/media/video/s5p-fimc/fimc-core.h

  drivers/media/video/s5p-fimc/fimc-reg.h

  drivers/media/video/s5p-fimc/regs-fimc.h

4. User space interfaces

========================

4.1. Media device interface

The driver supports Media Controller API as defined at

http://http://linuxtv.org/downloads/v4l-dvb-apis/media_common.html

The media device driver name is "SAMSUNG S5P FIMC".

The purpose of this interface is to allow changing assignment of FIMC instances

to the SoC peripheral camera input at runtime and optionally to control internal

connections of the MIPI-CSIS device(s) to the FIMC entities.

The media device interface allows to configure the SoC for capturing image

data from the sensor through more than one FIMC instance (e.g. for simultaneous

viewfinder and still capture setup).

Reconfiguration is done by enabling/disabling media links created by the driver

during initialization. The internal device topology can be easily discovered

through media entity and links enumeration.

4.2. Memory-to-memory video node

V4L2 memory-to-memory interface at /dev/video? device node.  This is standalone

video device, it has no media pads. However please note the mem-to-mem and

capture video node operation on same FIMC instance is not allowed.  The driver

detects such cases but the applications should prevent them to avoid an

undefined behaviour.

4.3. Capture video node

The driver supports V4L2 Video Capture Interface as defined at:

http://linuxtv.org/downloads/v4l-dvb-apis/devices.html

At the capture and mem-to-mem video nodes only the multi-planar API is

supported. For more details see:

http://linuxtv.org/downloads/v4l-dvb-apis/planar-apis.html

4.4. Camera capture subdevs

Each FIMC instance exports a sub-device node (/dev/v4l-subdev?), a sub-device

node is also created per each available and enabled at the platform level

MIPI-CSI receiver device (currently up to two).

4.5. sysfs

In order to enable more precise camera pipeline control through the sub-device

API the driver creates a sysfs entry associated with "s5p-fimc-md" platform

device. The entry path is: /sys/platform/devices/s5p-fimc-md/subdev_conf_mode.

In typical use case there could be a following capture pipeline configuration:

sensor subdev -> mipi-csi subdev -> fimc subdev -> video node

When we configure these devices through sub-device API at user space, the

configuration flow must be from left to right, and the video node is

configured as last one.

When we don't use sub-device user space API the whole configuration of all

devices belonging to the pipeline is done at the video node driver.

The sysfs entry allows to instruct the capture node driver not to configure

the sub-devices (format, crop), to avoid resetting the subdevs' configuration

when the last configuration steps at the video node is performed.

For full sub-device control support (subdevs configured at user space before

starting streaming):

# echo "sub-dev" > /sys/platform/devices/s5p-fimc-md/subdev_conf_mode

For V4L2 video node control only (subdevs configured internally by the host

driver):

# echo "vid-dev" > /sys/platform/devices/s5p-fimc-md/subdev_conf_mode

This is a default option.

5. Device mapping to video and subdev device nodes

==================================================

There are associated two video device nodes with each device instance in

hardware - video capture and mem-to-mem and additionally a subdev node for

more precise FIMC capture subsystem control. In addition a separate v4l2

sub-device node is created per each MIPI-CSIS device.

How to find out which /dev/video? or /dev/v4l-subdev? is assigned to which

device?

You can either grep through the kernel log to find relevant information, i.e.

# dmesg | grep -i fimc

(note that udev, if present, might still have rearranged the video nodes),

or retrieve the information from /dev/media? with help of the media-ctl tool:

# media-ctl -p

6. Platform support

===================

The machine code (plat-s5p and arch/arm/mach-*) must select following options

CONFIG_S5P_DEV_FIMC0       mandatory

CONFIG_S5P_DEV_FIMC1  \

CONFIG_S5P_DEV_FIMC2  |    optional

CONFIG_S5P_DEV_FIMC3  |

CONFIG_S5P_SETUP_FIMC /

CONFIG_S5P_SETUP_MIPIPHY \

CONFIG_S5P_DEV_CSIS0     | optional for MIPI-CSI interface

CONFIG_S5P_DEV_CSIS1     /

Except that, relevant s5p_device_fimc? should be registered in the machine code

in addition to a "s5p-fimc-md" platform device to which the media device driver

is bound.  The "s5p-fimc-md" device instance is required even if only mem-to-mem

operation is used.

The description of sensor(s) attached to FIMC/MIPI-CSIS camera inputs should be

passed as the "s5p-fimc-md" device platform_data.  The platform data structure

is defined in file include/media/s5p_fimc.h.

7. Build

========

This driver depends on following config options:

PLAT_S5P,

PM_RUNTIME,

I2C,

REGULATOR,

VIDEO_V4L2_SUBDEV_API,

If the driver is built as a loadable kernel module (CONFIG_VIDEO_SAMSUNG_S5P_FIMC=m)

two modules are created (in addition to the core v4l2 modules): s5p-fimc.ko and

optional s5p-csis.ko (MIPI-CSI receiver subdev).