1 Soc-Camera Subsystem 2 ==================== 3 4Terminology 5----------- 6 7The following terms are used in this document: 8 - camera / camera device / camera sensor - a video-camera sensor chip, capable 9 of connecting to a variety of systems and interfaces, typically uses i2c for 10 control and configuration, and a parallel or a serial bus for data. 11 - camera host - an interface, to which a camera is connected. Typically a 12 specialised interface, present on many SoCs, e.g. PXA27x and PXA3xx, SuperH, 13 AVR32, i.MX27, i.MX31. 14 - camera host bus - a connection between a camera host and a camera. Can be 15 parallel or serial, consists of data and control lines, e.g. clock, vertical 16 and horizontal synchronization signals. 17 18Purpose of the soc-camera subsystem 19----------------------------------- 20 21The soc-camera subsystem initially provided a unified API between camera host 22drivers and camera sensor drivers. Later the soc-camera sensor API has been 23replaced with the V4L2 standard subdev API. This also made camera driver re-use 24with non-soc-camera hosts possible. The camera host API to the soc-camera core 25has been preserved. 26 27Soc-camera implements a V4L2 interface to the user, currently only the "mmap" 28method is supported by host drivers. However, the soc-camera core also provides 29support for the "read" method. 30 31The subsystem has been designed to support multiple camera host interfaces and 32multiple cameras per interface, although most applications have only one camera 33sensor. 34 35Existing drivers 36---------------- 37 38As of 3.7 there are seven host drivers in the mainline: atmel-isi.c, 39mx1_camera.c (broken, scheduled for removal), mx2_camera.c, mx3_camera.c, 40omap1_camera.c, pxa_camera.c, sh_mobile_ceu_camera.c, and multiple sensor 41drivers under drivers/media/i2c/soc_camera/. 42 43Camera host API 44--------------- 45 46A host camera driver is registered using the 47 48soc_camera_host_register(struct soc_camera_host *); 49 50function. The host object can be initialized as follows: 51 52 struct soc_camera_host *ici; 53 ici->drv_name = DRV_NAME; 54 ici->ops = &camera_host_ops; 55 ici->priv = pcdev; 56 ici->v4l2_dev.dev = &pdev->dev; 57 ici->nr = pdev->id; 58 59All camera host methods are passed in a struct soc_camera_host_ops: 60 61static struct soc_camera_host_ops camera_host_ops = { 62 .owner = THIS_MODULE, 63 .add = camera_add_device, 64 .remove = camera_remove_device, 65 .set_fmt = camera_set_fmt_cap, 66 .try_fmt = camera_try_fmt_cap, 67 .init_videobuf2 = camera_init_videobuf2, 68 .poll = camera_poll, 69 .querycap = camera_querycap, 70 .set_bus_param = camera_set_bus_param, 71 /* The rest of host operations are optional */ 72}; 73 74.add and .remove methods are called when a sensor is attached to or detached 75from the host. .set_bus_param is used to configure physical connection 76parameters between the host and the sensor. .init_videobuf2 is called by 77soc-camera core when a video-device is opened, the host driver would typically 78call vb2_queue_init() in this method. Further video-buffer management is 79implemented completely by the specific camera host driver. If the host driver 80supports non-standard pixel format conversion, it should implement a 81.get_formats and, possibly, a .put_formats operations. See below for more 82details about format conversion. The rest of the methods are called from 83respective V4L2 operations. 84 85Camera API 86---------- 87 88Sensor drivers can use struct soc_camera_link, typically provided by the 89platform, and used to specify to which camera host bus the sensor is connected, 90and optionally provide platform .power and .reset methods for the camera. This 91struct is provided to the camera driver via the I2C client device platform data 92and can be obtained, using the soc_camera_i2c_to_link() macro. Care should be 93taken, when using soc_camera_vdev_to_subdev() and when accessing struct 94soc_camera_device, using v4l2_get_subdev_hostdata(): both only work, when 95running on an soc-camera host. The actual camera driver operation is implemented 96using the V4L2 subdev API. Additionally soc-camera camera drivers can use 97auxiliary soc-camera helper functions like soc_camera_power_on() and 98soc_camera_power_off(), which switch regulators, provided by the platform and call 99board-specific power switching methods. soc_camera_apply_board_flags() takes 100camera bus configuration capability flags and applies any board transformations, 101e.g. signal polarity inversion. soc_mbus_get_fmtdesc() can be used to obtain a 102pixel format descriptor, corresponding to a certain media-bus pixel format code. 103soc_camera_limit_side() can be used to restrict beginning and length of a frame 104side, based on camera capabilities. 105 106VIDIOC_S_CROP and VIDIOC_S_FMT behaviour 107---------------------------------------- 108 109Above user ioctls modify image geometry as follows: 110 111VIDIOC_S_CROP: sets location and sizes of the sensor window. Unit is one sensor 112pixel. Changing sensor window sizes preserves any scaling factors, therefore 113user window sizes change as well. 114 115VIDIOC_S_FMT: sets user window. Should preserve previously set sensor window as 116much as possible by modifying scaling factors. If the sensor window cannot be 117preserved precisely, it may be changed too. 118 119In soc-camera there are two locations, where scaling and cropping can take 120place: in the camera driver and in the host driver. User ioctls are first passed 121to the host driver, which then generally passes them down to the camera driver. 122It is more efficient to perform scaling and cropping in the camera driver to 123save camera bus bandwidth and maximise the framerate. However, if the camera 124driver failed to set the required parameters with sufficient precision, the host 125driver may decide to also use its own scaling and cropping to fulfill the user's 126request. 127 128Camera drivers are interfaced to the soc-camera core and to host drivers over 129the v4l2-subdev API, which is completely functional, it doesn't pass any data. 130Therefore all camera drivers shall reply to .g_fmt() requests with their current 131output geometry. This is necessary to correctly configure the camera bus. 132.s_fmt() and .try_fmt() have to be implemented too. Sensor window and scaling 133factors have to be maintained by camera drivers internally. According to the 134V4L2 API all capture drivers must support the VIDIOC_CROPCAP ioctl, hence we 135rely on camera drivers implementing .cropcap(). If the camera driver does not 136support cropping, it may choose to not implement .s_crop(), but to enable 137cropping support by the camera host driver at least the .g_crop method must be 138implemented. 139 140User window geometry is kept in .user_width and .user_height fields in struct 141soc_camera_device and used by the soc-camera core and host drivers. The core 142updates these fields upon successful completion of a .s_fmt() call, but if these 143fields change elsewhere, e.g. during .s_crop() processing, the host driver is 144responsible for updating them. 145 146Format conversion 147----------------- 148 149V4L2 distinguishes between pixel formats, as they are stored in memory, and as 150they are transferred over a media bus. Soc-camera provides support to 151conveniently manage these formats. A table of standard transformations is 152maintained by soc-camera core, which describes, what FOURCC pixel format will 153be obtained, if a media-bus pixel format is stored in memory according to 154certain rules. E.g. if MEDIA_BUS_FMT_YUYV8_2X8 data is sampled with 8 bits per 155sample and stored in memory in the little-endian order with no gaps between 156bytes, data in memory will represent the V4L2_PIX_FMT_YUYV FOURCC format. These 157standard transformations will be used by soc-camera or by camera host drivers to 158configure camera drivers to produce the FOURCC format, requested by the user, 159using the VIDIOC_S_FMT ioctl(). Apart from those standard format conversions, 160host drivers can also provide their own conversion rules by implementing a 161.get_formats and, if required, a .put_formats methods. 162 163-- 164Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de> 165