RFCl libv4l2 plugin API

Hi All,

RFC: a plugin API for libv4l2
=============================

During the Plumbers conference in 2009 various parties expresse interest in adding a plugin API to libv4l2. Some hardware can do some processing steps in hardware, but this needs to be setup from userspace and sometimes still need some regulation from userspace as streaming happens, hardware specific libv4l2 plugins could be a solution here.

During the v4l summit in Helsinki this was discussed further and a simple and very generic plugin API was pitched. This is a first draft specification for this API.

Basic libv4l2 principles
------------------------

The basic unit libv4l2's API deals with is a /dev/video node represented by a fd. A libv4l2 plugin will sit in between libv4l2 itself and the actual /dev/video device node a fd refers to. It will be called each time libv4l2 wants to do an operation (read/write/ioctl/mmap/munmap) on the actual /dev/video node in question. When called the plugin can then choose to do one of the following:
1. Pass the call unmodified to the fd, and return the return value unmodifed (iow do nothing)
2. Modify some arguments in the call and pass it through
3. Modify the return(ed) value(s) of a passed through call
4. Not do any operation on the fd at all but instead completely fake it (which opens the possibility for "fake" v4l devices)

Note that:
1. A plugin may decide between 1. - 4. on a per call basis depending on the operations arguments and / or state. This esp. is important with the ioctl operation where a plugin may want to intercept some ioctls but not all
2. If a plugin always wants to pass all calls through for a certain operation, it is allowed to simply not define a callback function for that operation
3. As libv4l2 does some "faking" itself esp. when it comes to format conversion a single libv4l2 call may result in multiple calls to the plugin, or to none at all.

So to summarize the above: Anything goes :)

libv4l2 plugin loading
----------------------

A libv4l2 plugin is a .so file which lives under /usr/lib[64]/libv4l/plugins. This .so file defines a single symbol: libv4l2_plugin. This symbol is a (constant) libv4l_plugin_data struct, which contains pointers to all the callbacks the plugin wants to install.

libv4l2 plugin loading begins with the application using libv4l2 doing a v4l2_open() for example v4l2_open("foobar", O_RDWR); When this happens libv4l2 will dlopen all files found under /usr/lib[64]/libv4l/plugins 1 at a time and call open callback passing through the applications parameters unmodified. If a plugin is relevant for the specified device node, it can indicate so by returning a value other then -1 from the open callback (usually the result of an actual open call on the device node). If it is not relevant for this specific video node it should return -1.

As soon as a plugin returns another value then -1 plugin loading stops and that plugins callbacks are called for all operations on the /dev/video# node in question. This means that plugins cannot be stacked!

Note that a plugin is bound to a specific fd, this way if there is for example a system with a SOC connected camera and an external usb camera, the SOC specific plugin will only be involved when an application is talking to the SOC connected camera and not when the app talks to the USB webcam.

Note that a plugin is bound to a specific fd not to a device node! IOW it is possible for a plugin to return not -1 from its open callback when called on /dev/video0 with O_RDWR, and to return -1 when called on /dev/video0 with O_RDONLY. Then its callbacks will only get called when operations are done on the video node though the RW fd, and not when operations are done on the same node through the RD_ONLY fd. This is not a good idea! It is only meant to emphasize the binding of a plugin to the fd, rather then to a specific device node.

libv4l2 plugins and private data
--------------------------------

libv4l2 plugins should *never* use any global variables. All data should be bound to the specific fd to which the plugin is bound. This ensures that for example a plugin for a specific type of usb webcam will also work when 2 identical cameras are plugged into a system (and both are used from the same process).

A libv4l2 plugin can register plugin private data using:
void libv4l2_set_plugindata(int fd, void *plugin_data);

And can get this data out of libv4l2 again inside a callback using:
void *libv4l2_get_plugindata(int fd);

Note that a plugin should call libv4l2_set_plugindata only once per fd ! Calling it a second time will overwrite the previous value. The logical place to use libv4l2_set_plugindata is from the plugin's open callback.

libv4l2-plugin.h
----------------

/* Plugin callback function struct */

struct libv4l2_plugin_data {
	int (*open)(const char *file, int oflag, ...);
	int (*close)(int fd);
	int (*ioctl)(int fd, unsigned long int request, ...);
	ssize_t (*read)(int fd, void *buffer, size_t n);
	void *(*mmap)(void *start, size_t length, int prot, int flags,
			int fd, int64_t offset);
	/* Note as munmap has no fd argument, defining a callback for munmap
	   will result in it getting called for *any* call to v4l2_munmap. So
	   if a plugin defines a callback for munmap (because for example it
	   returns fake mmap buffers from its mmap callback). Then it must
	   keep track of the addresses at which these buffers live and their
	   size and check the munmap arguments to see if the munmap call was
	   meant for it. */
	int (*munmap)(void *_start, size_t length);
};

/* Plugin utility functions */
void libv4l2_set_plugindata(int fd, void *plugin_data); void *libv4l2_get_plugindata(int fd);

Regards,

Hans

Hi Hans,

Thanks for the RFC!

I'd have a few comments and questions.

The coding style for libv4l hasn't been defined as far as I understand. Should kernel coding style be assumed, or something else?

Plugin internal data bound to file descriptors make probably sense for webcams, but we can have more complex setups than that. For example the OMAP 3 ISP driver. Several video nodes may be used to capture video, and there are dependencies between the nodes.

Essentially this kind of plugin can handle one media device at a time, not a V4L2 device. For webcams etc. this would still be the same since libv4l isn't aware of media devices.

However, the application is a V4L2 application which works on V4L2 devices and may well be unaware of the media device. I don't think this is _necessarily_ a problem since the plugin can do whatever it sees fit for user's request; it could even open another video node.

For example, there is likely one device node a general purpose application is interested in in the OMAP 3 ISP driver.

Obviously, there can be only one of this kind of media device control plugins at a time. Other plugins should have their say before that, so that the device appears a regular V4L2 device for those plugins as well.

The order in which the plugins are executed matters also if they do image processing. The result may well be different.

Perhaps it'd be good to experiment with one plugin per device first? :-)

What do you think?

Regards,

Sakari Ailus
[email protected]