From a47ddf1425554ca0b1e9b16b20a9d631e5daaaa8 Mon Sep 17 00:00:00 2001 From: Hans Verkuil Date: Fri, 19 Dec 2008 10:20:22 -0300 Subject: [PATCH] V4L/DVB (9943): v4l2: document video_device. Add the missing video_device documentation to v4l2-framework.txt. Signed-off-by: Hans Verkuil Signed-off-by: Mauro Carvalho Chehab --- Documentation/video4linux/v4l2-framework.txt | 160 ++++++++++++++++++++++++++- 1 file changed, 159 insertions(+), 1 deletion(-) diff --git a/Documentation/video4linux/v4l2-framework.txt b/Documentation/video4linux/v4l2-framework.txt index 60eaf54..eeae76c 100644 --- a/Documentation/video4linux/v4l2-framework.txt +++ b/Documentation/video4linux/v4l2-framework.txt @@ -86,6 +86,9 @@ to v4l2_dev. Registration will also set v4l2_dev->name to a value derived from dev (driver name followed by the bus_id, to be precise). You may change the name after registration if you want. +The first 'dev' argument is normally the struct device pointer of a pci_dev, +usb_device or platform_device. + You unregister with: v4l2_device_unregister(struct v4l2_device *v4l2_dev); @@ -359,4 +362,159 @@ Both functions return NULL if something went wrong. struct video_device ------------------- -Not yet documented. +The actual device nodes in the /dev directory are created using the +video_device struct (v4l2-dev.h). This struct can either be allocated +dynamically or embedded in a larger struct. + +To allocate it dynamically use: + + struct video_device *vdev = video_device_alloc(); + + if (vdev == NULL) + return -ENOMEM; + + vdev->release = video_device_release; + +If you embed it in a larger struct, then you must set the release() +callback to your own function: + + struct video_device *vdev = &my_vdev->vdev; + + vdev->release = my_vdev_release; + +The release callback must be set and it is called when the last user +of the video device exits. + +The default video_device_release() callback just calls kfree to free the +allocated memory. + +You should also set these fields: + +- parent: set to the parent device (same device as was used to register + v4l2_device). +- name: set to something descriptive and unique. +- fops: set to the file_operations struct. +- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance + (highly recommended to use this and it might become compulsory in the + future!), then set this to your v4l2_ioctl_ops struct. + +If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to +__video_ioctl2 or .ioctl to video_ioctl2 in your file_operations struct. + + +video_device registration +------------------------- + +Next you register the video device: this will create the character device +for you. + + err = video_register_device(vdev, VFL_TYPE_GRABBER, -1); + if (err) { + video_device_release(vdev); // or kfree(my_vdev); + return err; + } + +Which device is registered depends on the type argument. The following +types exist: + +VFL_TYPE_GRABBER: videoX for video input/output devices +VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext) +VFL_TYPE_RADIO: radioX for radio tuners +VFL_TYPE_VTX: vtxX for teletext devices (deprecated, don't use) + +The last argument gives you a certain amount of control over the device +kernel number used (i.e. the X in videoX). Normally you will pass -1 to +let the v4l2 framework pick the first free number. But if a driver creates +many devices, then it can be useful to have different video devices in +separate ranges. For example, video capture devices start at 0, video +output devices start at 16. + +So you can use the last argument to specify a minimum kernel number and +the v4l2 framework will try to pick the first free number that is equal +or higher to what you passed. If that fails, then it will just pick the +first free number. + +Whenever a device node is created some attributes are also created for you. +If you look in /sys/class/video4linux you see the devices. Go into e.g. +video0 and you will see 'name' and 'index' attributes. The 'name' attribute +is the 'name' field of the video_device struct. The 'index' attribute is +a device node index that can be assigned by the driver, or that is calculated +for you. + +If you call video_register_device(), then the index is just increased by +1 for each device node you register. The first video device node you register +always starts off with 0. + +Alternatively you can call video_register_device_index() which is identical +to video_register_device(), but with an extra index argument. Here you can +pass a specific index value (between 0 and 31) that should be used. + +Users can setup udev rules that utilize the index attribute to make fancy +device names (e.g. 'mpegX' for MPEG video capture device nodes). + +After the device was successfully registered, then you can use these fields: + +- vfl_type: the device type passed to video_register_device. +- minor: the assigned device minor number. +- num: the device kernel number (i.e. the X in videoX). +- index: the device index number (calculated or set explicitly using + video_register_device_index). + +If the registration failed, then you need to call video_device_release() +to free the allocated video_device struct, or free your own struct if the +video_device was embedded in it. The vdev->release() callback will never +be called if the registration failed, nor should you ever attempt to +unregister the device if the registration failed. + + +video_device cleanup +-------------------- + +When the video device nodes have to be removed, either during the unload +of the driver or because the USB device was disconnected, then you should +unregister them: + + video_unregister_device(vdev); + +This will remove the device nodes from sysfs (causing udev to remove them +from /dev). + +After video_unregister_device() returns no new opens can be done. + +However, in the case of USB devices some application might still have one +of these device nodes open. You should block all new accesses to read, +write, poll, etc. except possibly for certain ioctl operations like +queueing buffers. + +When the last user of the video device node exits, then the vdev->release() +callback is called and you can do the final cleanup there. + + +video_device helper functions +----------------------------- + +There are a few useful helper functions: + +You can set/get driver private data in the video_device struct using: + +void *video_get_drvdata(struct video_device *dev); +void video_set_drvdata(struct video_device *dev, void *data); + +Note that you can safely call video_set_drvdata() before calling +video_register_device(). + +And this function: + +struct video_device *video_devdata(struct file *file); + +returns the video_device belonging to the file struct. + +The final helper function combines video_get_drvdata with +video_devdata: + +void *video_drvdata(struct file *file); + +You can go from a video_device struct to the v4l2_device struct using: + +struct v4l2_device *v4l2_dev = dev_get_drvdata(vdev->parent); + -- 2.7.4