165 lines
7.7 KiB
Plaintext
165 lines
7.7 KiB
Plaintext
Soc-Camera Subsystem
|
|
====================
|
|
|
|
Terminology
|
|
-----------
|
|
|
|
The following terms are used in this document:
|
|
- camera / camera device / camera sensor - a video-camera sensor chip, capable
|
|
of connecting to a variety of systems and interfaces, typically uses i2c for
|
|
control and configuration, and a parallel or a serial bus for data.
|
|
- camera host - an interface, to which a camera is connected. Typically a
|
|
specialised interface, present on many SoCs, e.g. PXA27x and PXA3xx, SuperH,
|
|
AVR32, i.MX27, i.MX31.
|
|
- camera host bus - a connection between a camera host and a camera. Can be
|
|
parallel or serial, consists of data and control lines, e.g. clock, vertical
|
|
and horizontal synchronization signals.
|
|
|
|
Purpose of the soc-camera subsystem
|
|
-----------------------------------
|
|
|
|
The soc-camera subsystem initially provided a unified API between camera host
|
|
drivers and camera sensor drivers. Later the soc-camera sensor API has been
|
|
replaced with the V4L2 standard subdev API. This also made camera driver re-use
|
|
with non-soc-camera hosts possible. The camera host API to the soc-camera core
|
|
has been preserved.
|
|
|
|
Soc-camera implements a V4L2 interface to the user, currently only the "mmap"
|
|
method is supported by host drivers. However, the soc-camera core also provides
|
|
support for the "read" method.
|
|
|
|
The subsystem has been designed to support multiple camera host interfaces and
|
|
multiple cameras per interface, although most applications have only one camera
|
|
sensor.
|
|
|
|
Existing drivers
|
|
----------------
|
|
|
|
As of 3.7 there are seven host drivers in the mainline: atmel-isi.c,
|
|
mx1_camera.c (broken, scheduled for removal), mx2_camera.c, mx3_camera.c,
|
|
omap1_camera.c, pxa_camera.c, sh_mobile_ceu_camera.c, and multiple sensor
|
|
drivers under drivers/media/i2c/soc_camera/.
|
|
|
|
Camera host API
|
|
---------------
|
|
|
|
A host camera driver is registered using the
|
|
|
|
soc_camera_host_register(struct soc_camera_host *);
|
|
|
|
function. The host object can be initialized as follows:
|
|
|
|
struct soc_camera_host *ici;
|
|
ici->drv_name = DRV_NAME;
|
|
ici->ops = &camera_host_ops;
|
|
ici->priv = pcdev;
|
|
ici->v4l2_dev.dev = &pdev->dev;
|
|
ici->nr = pdev->id;
|
|
|
|
All camera host methods are passed in a struct soc_camera_host_ops:
|
|
|
|
static struct soc_camera_host_ops camera_host_ops = {
|
|
.owner = THIS_MODULE,
|
|
.add = camera_add_device,
|
|
.remove = camera_remove_device,
|
|
.set_fmt = camera_set_fmt_cap,
|
|
.try_fmt = camera_try_fmt_cap,
|
|
.init_videobuf2 = camera_init_videobuf2,
|
|
.poll = camera_poll,
|
|
.querycap = camera_querycap,
|
|
.set_bus_param = camera_set_bus_param,
|
|
/* The rest of host operations are optional */
|
|
};
|
|
|
|
.add and .remove methods are called when a sensor is attached to or detached
|
|
from the host. .set_bus_param is used to configure physical connection
|
|
parameters between the host and the sensor. .init_videobuf2 is called by
|
|
soc-camera core when a video-device is opened, the host driver would typically
|
|
call vb2_queue_init() in this method. Further video-buffer management is
|
|
implemented completely by the specific camera host driver. If the host driver
|
|
supports non-standard pixel format conversion, it should implement a
|
|
.get_formats and, possibly, a .put_formats operations. See below for more
|
|
details about format conversion. The rest of the methods are called from
|
|
respective V4L2 operations.
|
|
|
|
Camera API
|
|
----------
|
|
|
|
Sensor drivers can use struct soc_camera_link, typically provided by the
|
|
platform, and used to specify to which camera host bus the sensor is connected,
|
|
and optionally provide platform .power and .reset methods for the camera. This
|
|
struct is provided to the camera driver via the I2C client device platform data
|
|
and can be obtained, using the soc_camera_i2c_to_link() macro. Care should be
|
|
taken, when using soc_camera_vdev_to_subdev() and when accessing struct
|
|
soc_camera_device, using v4l2_get_subdev_hostdata(): both only work, when
|
|
running on an soc-camera host. The actual camera driver operation is implemented
|
|
using the V4L2 subdev API. Additionally soc-camera camera drivers can use
|
|
auxiliary soc-camera helper functions like soc_camera_power_on() and
|
|
soc_camera_power_off(), which switch regulators, provided by the platform and call
|
|
board-specific power switching methods. soc_camera_apply_board_flags() takes
|
|
camera bus configuration capability flags and applies any board transformations,
|
|
e.g. signal polarity inversion. soc_mbus_get_fmtdesc() can be used to obtain a
|
|
pixel format descriptor, corresponding to a certain media-bus pixel format code.
|
|
soc_camera_limit_side() can be used to restrict beginning and length of a frame
|
|
side, based on camera capabilities.
|
|
|
|
VIDIOC_S_CROP and VIDIOC_S_FMT behaviour
|
|
----------------------------------------
|
|
|
|
Above user ioctls modify image geometry as follows:
|
|
|
|
VIDIOC_S_CROP: sets location and sizes of the sensor window. Unit is one sensor
|
|
pixel. Changing sensor window sizes preserves any scaling factors, therefore
|
|
user window sizes change as well.
|
|
|
|
VIDIOC_S_FMT: sets user window. Should preserve previously set sensor window as
|
|
much as possible by modifying scaling factors. If the sensor window cannot be
|
|
preserved precisely, it may be changed too.
|
|
|
|
In soc-camera there are two locations, where scaling and cropping can taks
|
|
place: in the camera driver and in the host driver. User ioctls are first passed
|
|
to the host driver, which then generally passes them down to the camera driver.
|
|
It is more efficient to perform scaling and cropping in the camera driver to
|
|
save camera bus bandwidth and maximise the framerate. However, if the camera
|
|
driver failed to set the required parameters with sufficient precision, the host
|
|
driver may decide to also use its own scaling and cropping to fulfill the user's
|
|
request.
|
|
|
|
Camera drivers are interfaced to the soc-camera core and to host drivers over
|
|
the v4l2-subdev API, which is completely functional, it doesn't pass any data.
|
|
Therefore all camera drivers shall reply to .g_fmt() requests with their current
|
|
output geometry. This is necessary to correctly configure the camera bus.
|
|
.s_fmt() and .try_fmt() have to be implemented too. Sensor window and scaling
|
|
factors have to be maintained by camera drivers internally. According to the
|
|
V4L2 API all capture drivers must support the VIDIOC_CROPCAP ioctl, hence we
|
|
rely on camera drivers implementing .cropcap(). If the camera driver does not
|
|
support cropping, it may choose to not implement .s_crop(), but to enable
|
|
cropping support by the camera host driver at least the .g_crop method must be
|
|
implemented.
|
|
|
|
User window geometry is kept in .user_width and .user_height fields in struct
|
|
soc_camera_device and used by the soc-camera core and host drivers. The core
|
|
updates these fields upon successful completion of a .s_fmt() call, but if these
|
|
fields change elsewhere, e.g. during .s_crop() processing, the host driver is
|
|
responsible for updating them.
|
|
|
|
Format conversion
|
|
-----------------
|
|
|
|
V4L2 distinguishes between pixel formats, as they are stored in memory, and as
|
|
they are transferred over a media bus. Soc-camera provides support to
|
|
conveniently manage these formats. A table of standard transformations is
|
|
maintained by soc-camera core, which describes, what FOURCC pixel format will
|
|
be obtained, if a media-bus pixel format is stored in memory according to
|
|
certain rules. E.g. if V4L2_MBUS_FMT_YUYV8_2X8 data is sampled with 8 bits per
|
|
sample and stored in memory in the little-endian order with no gaps between
|
|
bytes, data in memory will represent the V4L2_PIX_FMT_YUYV FOURCC format. These
|
|
standard transformations will be used by soc-camera or by camera host drivers to
|
|
configure camera drivers to produce the FOURCC format, requested by the user,
|
|
using the VIDIOC_S_FMT ioctl(). Apart from those standard format conversions,
|
|
host drivers can also provide their own conversion rules by implementing a
|
|
.get_formats and, if required, a .put_formats methods.
|
|
|
|
--
|
|
Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
|