This is the second article in the LWN series on writing drivers for the
Video4Linux2 kernel interface; those who have not yet seen the introductory article
wish to start there. This installment will look at the overall structure
of a Video4Linux driver and the device registration process.
Before starting, it is worth noting that there are two resources which will
prove invaluable for anybody working with video drivers:
- The V4L2 API
Specification. This document covers the API from the user-space
point of view, but, to a great extent, V4L2 drivers implement that API
directly. So most of the structures are the same, and the semantics
of the V4L2 calls are clearly laid out. Print a copy (consider
cutting out the Free Documentation License text to save trees) and
keep it somewhere within easy reach.
- The "vivi" driver found in the kernel source as
drivers/media/video/vivi.c. It is a virtual driver, in that
it generates test patterns and does not actually interface to any
hardware. As such, it serves as a relatively clear illustration of
how V4L2 drivers should be written.
To start, every V4L2 driver must include the requisite header file:
Much of the needed information is there. When digging through the headers
as a driver author, however, you'll also want to have a look at
include/media/v4l2-dev.h, which defines many of the structures you'll
be working with.
A video driver will probably have sections which deal with the PCI or USB
bus (for example); we'll not spend much time on that part of the driver
here. There is often an internal i2c interface, which will be
examined later on in this article series. Then, there is the interface to
the V4L2 subsystem. That interface is built around struct
video_device, which represents a V4L2 device. Covering everything
that goes into this structure will be the topic of several articles; here
we'll just have an overview.
The name field of struct video_device is a name for the
type of device; it will appear in kernel log messages and in sysfs. The
name usually matches the name of the driver.
There are two fields to describe what type of device is being represented.
The first (type) looks like a holdover from the Video4Linux1 API;
it can have one of four values:
- VFL_TYPE_GRABBER indicates a frame grabber device - including
cameras, tuners, and such.
- VFL_TYPE_VBI is for devices which pull information
transmitted during the video blanking interval.
- VFL_TYPE_RADIO for radio devices.
- VFL_TYPE_VTX for videotext devices.
If your device can perform more than one of the above functions, a separate
V4L2 device should be registered for each of the supported functions. In
V4L2, however, any of the registered devices can be called upon to function
in any of the supported modes. What it comes down to is that, for V4L2,
there is really only need for a single device, but compatibility with the
older Video4Linux API requires that individual devices be registered for
The second field, called type2, is a bitmask describing the
device's capabilities in more detail. It can contain any of the following
- VID_TYPE_CAPTURE: the device can capture video data.
- VID_TYPE_TUNER: it can tune to different frequencies.
- VID_TYPE_TELETEXT: it can grab teletext data.
- VID_TYPE_OVERLAY: it can overlay video data directly
into the frame buffer.
- VID_TYPE_CHROMAKEY: a special form of overlay capability
where the video data is only displayed where the underlying
frame buffer contains pixels of a specific color.
- VID_TYPE_CLIPPING: it can clip overlay data.
- VID_TYPE_FRAMERAM: it uses memory located in the frame buffer
- VID_TYPE_SCALES: it can scale video data.
- VID_TYPE_MONOCHROME: it is a monochrome-only device.
- VID_TYPE_SUBCAPTURE: it can capture sub-areas of the image.
- VID_TYPE_MPEG_DECODER: it can decode MPEG streams.
- VID_TYPE_MPEG_ENCODER: it can encode MPEG streams.
- VID_TYPE_MJPEG_DECODER: it can decode MJPEG streams.
- VID_TYPE_MJPEG_ENCODER: it can encode MJPEG streams.
Another field initialized by all V4L2 drivers is minor, which is
the desired minor number for the device. Usually this field will be set to
-1, which causes the Video4Linux subsystem to allocate a minor number at
There are also three distinct sets of function pointers found within
struct video_device. The first, consisting of a single function,
is the release() method. If a device lacks a release()
function, the kernel will complain (your editor was amused to note that it
refers offending programmers to an LWN article). The release()
function is important: for various reasons, references to a
video_device structure can remain long after that last video
application has closed its file descriptor. Those references can remain
after the device has been unregistered. For this reason, it is not safe to
free the structure until the release() method has been called.
So, often, this function consists of a simple kfree() call.
The video_device structure contains within it a
file_operations structure with the usual function pointers. Video
drivers will always need open() and release() operations;
note that this release() is called whenever the device is
closed, not when it can be freed as with the other function with the same
name described above. There will often be a read() or
write() method, depending on whether the device performs input or
output; note, however, that for streaming video devices, there are other
ways of transferring data. Most devices which handle streaming video data
will need to implement poll() and mmap(). And
every V4l2 device needs an ioctl() method - but they can
use video_ioctl2(), which is provided by the V4L2 subsystem.
The third set of methods, stored in the video_device structure
itself, makes up the core of the V4L2 API. There are several dozen of
them, handling various device configuration operations, streaming I/O, and
Finally, a useful field to know from the beginning is debug.
Setting it to either (or both - it's a bitmask) of V4L2_DEBUG_IOCTL and
V4L2_DEBUG_IOCTL_ARG will yield a fair amount of debugging output
which can help a befuddled programmer figure out why a driver and an
application are failing to understand each other.
Video device registration
Once the video_device structure has been set up, it should be
int video_register_device(struct video_device *vfd, int type, int nr);
Here, vfd is the device structure, type is the same value
found in its type field, and nr is, again, the desired
minor number (or -1 for dynamic allocation). The return value should be
zero; a negative error code indicates that something went badly wrong. As
always, one should be aware that the device's methods can be called
immediately once the device is registered; do not call
video_register_device() until everything is ready to go.
A device can be unregistered with:
void video_unregister_device(struct video_device *vfd);
Stay tuned for the next article in this series, which will begin to look at
the implementation of some of these methods.
open() and release()
Every V4L2 device will need an open() method, which will have the
int (*open)(struct inode *inode, struct file *filp);
The first thing an open() method will normally do is to locate an
internal device corresponding to the given inode; this is done by
keying on the minor number stored in inode. A certain amount of
initialization can be performed; this can also be a good time to power up
the hardware if it has a power-down option.
The V4L2 specification defines some conventions which are relevant here.
One is that, by design, all V4L2 devices can have multiple open file
descriptors at any given time. The purpose here is to allow one
application to display (or generate) video data while another one, perhaps,
tweaks control values. So, while certain V4L2 operations (actually reading
and writing video data, in particular) can be made
exclusive to a single file descriptor, the device as a whole should support
multiple open descriptors.
Another convention worth mentioning is that the open() method
should not, in general, make changes to the operating parameters currently
set in the hardware. It should be possible to run a command-line program
which configures a camera according to a certain set of desires
(resolution, video format, etc.), then run an entirely separate application
to, for example, capture a frame from the camera. This mode would not work
if the camera's settings were reset in the middle, so a V4L2 driver should
endeavor to keep existing settings until an application explicitly resets
The release() method performs any needed cleanup. Since video
devices can have multiple open file descriptors, release() will
need to decrement a counter and check before doing anything radical. If
the just-closed file descriptor was being used to transfer data, it may
necessary to shut down the DMA engine and perform other cleanups.
The next installment in this series will start into the long process of
querying device capabilities and configuring operating modes. Stay tuned.
to post comments)