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diff --git a/Documentation/video4linux/soc-camera.txt b/Documentation/video4linux/soc-camera.txt new file mode 100644 index 00000000..3f87c7da --- /dev/null +++ b/Documentation/video4linux/soc-camera.txt @@ -0,0 +1,160 @@ + 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 provides a unified API between camera host drivers and +camera sensor drivers. It implements a V4L2 interface to the user, currently +only the mmap method is supported. + +This subsystem has been written to connect drivers for System-on-Chip (SoC) +video capture interfaces with drivers for CMOS camera sensor chips to enable +the reuse of sensor drivers with various hosts. 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 2.6.27-rc4 there are two host drivers in the mainline: pxa_camera.c for +PXA27x SoCs and sh_mobile_ceu_camera.c for SuperH SoCs, and four sensor drivers: +mt9m001.c, mt9m111.c, mt9v022.c and a generic soc_camera_platform.c driver. This +list is not supposed to be updated, look for more examples in your tree. + +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: + +static struct soc_camera_host pxa_soc_camera_host = { + .drv_name = PXA_CAM_DRV_NAME, + .ops = &pxa_soc_camera_host_ops, +}; + +All camera host methods are passed in a struct soc_camera_host_ops: + +static struct soc_camera_host_ops pxa_soc_camera_host_ops = { + .owner = THIS_MODULE, + .add = pxa_camera_add_device, + .remove = pxa_camera_remove_device, + .suspend = pxa_camera_suspend, + .resume = pxa_camera_resume, + .set_fmt_cap = pxa_camera_set_fmt_cap, + .try_fmt_cap = pxa_camera_try_fmt_cap, + .init_videobuf = pxa_camera_init_videobuf, + .reqbufs = pxa_camera_reqbufs, + .poll = pxa_camera_poll, + .querycap = pxa_camera_querycap, + .try_bus_param = pxa_camera_try_bus_param, + .set_bus_param = pxa_camera_set_bus_param, +}; + +.add and .remove methods are called when a sensor is attached to or detached +from the host, apart from performing host-internal tasks they shall also call +sensor driver's .init and .release methods respectively. .suspend and .resume +methods implement host's power-management functionality and its their +responsibility to call respective sensor's methods. .try_bus_param and +.set_bus_param are used to negotiate physical connection parameters between the +host and the sensor. .init_videobuf is called by soc-camera core when a +video-device is opened, further video-buffer management is implemented completely +by the specific camera host driver. 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 arbitrarily provide platform .power and .reset methods for the camera. +soc_camera_device_register() and soc_camera_device_unregister() functions are +used to add a sensor driver to or remove one from the system. The registration +function takes a pointer to struct soc_camera_device as the only parameter. +This struct can be initialized as follows: + + /* link to driver operations */ + icd->ops = &mt9m001_ops; + /* link to the underlying physical (e.g., i2c) device */ + icd->control = &client->dev; + /* window geometry */ + icd->x_min = 20; + icd->y_min = 12; + icd->x_current = 20; + icd->y_current = 12; + icd->width_min = 48; + icd->width_max = 1280; + icd->height_min = 32; + icd->height_max = 1024; + icd->y_skip_top = 1; + /* camera bus ID, typically obtained from platform data */ + icd->iface = icl->bus_id; + +struct soc_camera_ops provides .probe and .remove methods, which are called by +the soc-camera core, when a camera is matched against or removed from a camera +host bus, .init, .release, .suspend, and .resume are called from the camera host +driver as discussed above. Other members of this struct provide respective V4L2 +functionality. + +struct soc_camera_device also links to an array of struct soc_camera_data_format, +listing pixel formats, supported by the camera. + +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. + +-- +Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de> |