V4L/DVB (9943): v4l2: document video_device.
Add the missing video_device documentation to v4l2-framework.txt. Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
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Hans Verkuil
Mauro Carvalho Chehab
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@ -86,6 +86,9 @@ to v4l2_dev. Registration will also set v4l2_dev->name to a value derived from
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dev (driver name followed by the bus_id, to be precise). You may change the
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name after registration if you want.
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The first 'dev' argument is normally the struct device pointer of a pci_dev,
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usb_device or platform_device.
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You unregister with:
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v4l2_device_unregister(struct v4l2_device *v4l2_dev);
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@ -359,4 +362,159 @@ Both functions return NULL if something went wrong.
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struct video_device
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-------------------
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Not yet documented.
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The actual device nodes in the /dev directory are created using the
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video_device struct (v4l2-dev.h). This struct can either be allocated
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dynamically or embedded in a larger struct.
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To allocate it dynamically use:
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struct video_device *vdev = video_device_alloc();
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if (vdev == NULL)
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return -ENOMEM;
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vdev->release = video_device_release;
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If you embed it in a larger struct, then you must set the release()
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callback to your own function:
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struct video_device *vdev = &my_vdev->vdev;
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vdev->release = my_vdev_release;
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The release callback must be set and it is called when the last user
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of the video device exits.
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The default video_device_release() callback just calls kfree to free the
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allocated memory.
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You should also set these fields:
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- parent: set to the parent device (same device as was used to register
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v4l2_device).
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- name: set to something descriptive and unique.
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- fops: set to the file_operations struct.
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- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
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(highly recommended to use this and it might become compulsory in the
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future!), then set this to your v4l2_ioctl_ops struct.
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If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to
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__video_ioctl2 or .ioctl to video_ioctl2 in your file_operations struct.
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video_device registration
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-------------------------
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Next you register the video device: this will create the character device
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for you.
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err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
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if (err) {
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video_device_release(vdev); // or kfree(my_vdev);
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return err;
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}
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Which device is registered depends on the type argument. The following
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types exist:
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VFL_TYPE_GRABBER: videoX for video input/output devices
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VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
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VFL_TYPE_RADIO: radioX for radio tuners
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VFL_TYPE_VTX: vtxX for teletext devices (deprecated, don't use)
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The last argument gives you a certain amount of control over the device
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kernel number used (i.e. the X in videoX). Normally you will pass -1 to
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let the v4l2 framework pick the first free number. But if a driver creates
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many devices, then it can be useful to have different video devices in
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separate ranges. For example, video capture devices start at 0, video
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output devices start at 16.
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So you can use the last argument to specify a minimum kernel number and
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the v4l2 framework will try to pick the first free number that is equal
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or higher to what you passed. If that fails, then it will just pick the
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first free number.
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Whenever a device node is created some attributes are also created for you.
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If you look in /sys/class/video4linux you see the devices. Go into e.g.
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video0 and you will see 'name' and 'index' attributes. The 'name' attribute
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is the 'name' field of the video_device struct. The 'index' attribute is
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a device node index that can be assigned by the driver, or that is calculated
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for you.
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If you call video_register_device(), then the index is just increased by
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1 for each device node you register. The first video device node you register
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always starts off with 0.
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Alternatively you can call video_register_device_index() which is identical
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to video_register_device(), but with an extra index argument. Here you can
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pass a specific index value (between 0 and 31) that should be used.
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Users can setup udev rules that utilize the index attribute to make fancy
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device names (e.g. 'mpegX' for MPEG video capture device nodes).
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After the device was successfully registered, then you can use these fields:
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- vfl_type: the device type passed to video_register_device.
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- minor: the assigned device minor number.
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- num: the device kernel number (i.e. the X in videoX).
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- index: the device index number (calculated or set explicitly using
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video_register_device_index).
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If the registration failed, then you need to call video_device_release()
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to free the allocated video_device struct, or free your own struct if the
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video_device was embedded in it. The vdev->release() callback will never
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be called if the registration failed, nor should you ever attempt to
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unregister the device if the registration failed.
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video_device cleanup
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--------------------
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When the video device nodes have to be removed, either during the unload
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of the driver or because the USB device was disconnected, then you should
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unregister them:
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video_unregister_device(vdev);
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This will remove the device nodes from sysfs (causing udev to remove them
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from /dev).
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After video_unregister_device() returns no new opens can be done.
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However, in the case of USB devices some application might still have one
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of these device nodes open. You should block all new accesses to read,
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write, poll, etc. except possibly for certain ioctl operations like
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queueing buffers.
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When the last user of the video device node exits, then the vdev->release()
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callback is called and you can do the final cleanup there.
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video_device helper functions
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-----------------------------
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There are a few useful helper functions:
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You can set/get driver private data in the video_device struct using:
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void *video_get_drvdata(struct video_device *dev);
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void video_set_drvdata(struct video_device *dev, void *data);
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Note that you can safely call video_set_drvdata() before calling
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video_register_device().
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And this function:
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struct video_device *video_devdata(struct file *file);
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returns the video_device belonging to the file struct.
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The final helper function combines video_get_drvdata with
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video_devdata:
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void *video_drvdata(struct file *file);
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You can go from a video_device struct to the v4l2_device struct using:
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struct v4l2_device *v4l2_dev = dev_get_drvdata(vdev->parent);
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