161 lines
6.9 KiB
Plaintext
161 lines
6.9 KiB
Plaintext
Soc-Camera Subsystem
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====================
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Terminology
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-----------
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The following terms are used in this document:
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- camera / camera device / camera sensor - a video-camera sensor chip, capable
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of connecting to a variety of systems and interfaces, typically uses i2c for
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control and configuration, and a parallel or a serial bus for data.
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- camera host - an interface, to which a camera is connected. Typically a
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specialised interface, present on many SoCs, e.g., PXA27x and PXA3xx, SuperH,
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AVR32, i.MX27, i.MX31.
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- camera host bus - a connection between a camera host and a camera. Can be
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parallel or serial, consists of data and control lines, e.g., clock, vertical
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and horizontal synchronization signals.
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Purpose of the soc-camera subsystem
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-----------------------------------
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The soc-camera subsystem provides a unified API between camera host drivers and
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camera sensor drivers. It implements a V4L2 interface to the user, currently
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only the mmap method is supported.
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This subsystem has been written to connect drivers for System-on-Chip (SoC)
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video capture interfaces with drivers for CMOS camera sensor chips to enable
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the reuse of sensor drivers with various hosts. The subsystem has been designed
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to support multiple camera host interfaces and multiple cameras per interface,
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although most applications have only one camera sensor.
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Existing drivers
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----------------
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As of 2.6.27-rc4 there are two host drivers in the mainline: pxa_camera.c for
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PXA27x SoCs and sh_mobile_ceu_camera.c for SuperH SoCs, and four sensor drivers:
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mt9m001.c, mt9m111.c, mt9v022.c and a generic soc_camera_platform.c driver. This
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list is not supposed to be updated, look for more examples in your tree.
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Camera host API
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---------------
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A host camera driver is registered using the
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soc_camera_host_register(struct soc_camera_host *);
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function. The host object can be initialized as follows:
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static struct soc_camera_host pxa_soc_camera_host = {
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.drv_name = PXA_CAM_DRV_NAME,
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.ops = &pxa_soc_camera_host_ops,
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};
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All camera host methods are passed in a struct soc_camera_host_ops:
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static struct soc_camera_host_ops pxa_soc_camera_host_ops = {
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.owner = THIS_MODULE,
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.add = pxa_camera_add_device,
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.remove = pxa_camera_remove_device,
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.suspend = pxa_camera_suspend,
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.resume = pxa_camera_resume,
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.set_fmt_cap = pxa_camera_set_fmt_cap,
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.try_fmt_cap = pxa_camera_try_fmt_cap,
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.init_videobuf = pxa_camera_init_videobuf,
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.reqbufs = pxa_camera_reqbufs,
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.poll = pxa_camera_poll,
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.querycap = pxa_camera_querycap,
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.try_bus_param = pxa_camera_try_bus_param,
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.set_bus_param = pxa_camera_set_bus_param,
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};
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.add and .remove methods are called when a sensor is attached to or detached
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from the host, apart from performing host-internal tasks they shall also call
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sensor driver's .init and .release methods respectively. .suspend and .resume
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methods implement host's power-management functionality and its their
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responsibility to call respective sensor's methods. .try_bus_param and
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.set_bus_param are used to negotiate physical connection parameters between the
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host and the sensor. .init_videobuf is called by soc-camera core when a
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video-device is opened, further video-buffer management is implemented completely
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by the specific camera host driver. The rest of the methods are called from
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respective V4L2 operations.
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Camera API
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----------
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Sensor drivers can use struct soc_camera_link, typically provided by the
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platform, and used to specify to which camera host bus the sensor is connected,
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and arbitrarily provide platform .power and .reset methods for the camera.
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soc_camera_device_register() and soc_camera_device_unregister() functions are
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used to add a sensor driver to or remove one from the system. The registration
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function takes a pointer to struct soc_camera_device as the only parameter.
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This struct can be initialized as follows:
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/* link to driver operations */
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icd->ops = &mt9m001_ops;
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/* link to the underlying physical (e.g., i2c) device */
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icd->control = &client->dev;
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/* window geometry */
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icd->x_min = 20;
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icd->y_min = 12;
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icd->x_current = 20;
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icd->y_current = 12;
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icd->width_min = 48;
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icd->width_max = 1280;
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icd->height_min = 32;
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icd->height_max = 1024;
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icd->y_skip_top = 1;
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/* camera bus ID, typically obtained from platform data */
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icd->iface = icl->bus_id;
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struct soc_camera_ops provides .probe and .remove methods, which are called by
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the soc-camera core, when a camera is matched against or removed from a camera
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host bus, .init, .release, .suspend, and .resume are called from the camera host
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driver as discussed above. Other members of this struct provide respective V4L2
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functionality.
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struct soc_camera_device also links to an array of struct soc_camera_data_format,
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listing pixel formats, supported by the camera.
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VIDIOC_S_CROP and VIDIOC_S_FMT behaviour
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----------------------------------------
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Above user ioctls modify image geometry as follows:
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VIDIOC_S_CROP: sets location and sizes of the sensor window. Unit is one sensor
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pixel. Changing sensor window sizes preserves any scaling factors, therefore
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user window sizes change as well.
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VIDIOC_S_FMT: sets user window. Should preserve previously set sensor window as
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much as possible by modifying scaling factors. If the sensor window cannot be
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preserved precisely, it may be changed too.
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In soc-camera there are two locations, where scaling and cropping can taks
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place: in the camera driver and in the host driver. User ioctls are first passed
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to the host driver, which then generally passes them down to the camera driver.
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It is more efficient to perform scaling and cropping in the camera driver to
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save camera bus bandwidth and maximise the framerate. However, if the camera
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driver failed to set the required parameters with sufficient precision, the host
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driver may decide to also use its own scaling and cropping to fulfill the user's
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request.
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Camera drivers are interfaced to the soc-camera core and to host drivers over
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the v4l2-subdev API, which is completely functional, it doesn't pass any data.
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Therefore all camera drivers shall reply to .g_fmt() requests with their current
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output geometry. This is necessary to correctly configure the camera bus.
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.s_fmt() and .try_fmt() have to be implemented too. Sensor window and scaling
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factors have to be maintained by camera drivers internally. According to the
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V4L2 API all capture drivers must support the VIDIOC_CROPCAP ioctl, hence we
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rely on camera drivers implementing .cropcap(). If the camera driver does not
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support cropping, it may choose to not implement .s_crop(), but to enable
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cropping support by the camera host driver at least the .g_crop method must be
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implemented.
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User window geometry is kept in .user_width and .user_height fields in struct
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soc_camera_device and used by the soc-camera core and host drivers. The core
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updates these fields upon successful completion of a .s_fmt() call, but if these
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fields change elsewhere, e.g., during .s_crop() processing, the host driver is
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responsible for updating them.
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--
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Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
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