f5360bdc55
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
527 lines
19 KiB
Text
527 lines
19 KiB
Text
Overview of the V4L2 driver framework
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=====================================
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This text documents the various structures provided by the V4L2 framework and
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their relationships.
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Introduction
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------------
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The V4L2 drivers tend to be very complex due to the complexity of the
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hardware: most devices have multiple ICs, export multiple device nodes in
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/dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input
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(IR) devices.
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Especially the fact that V4L2 drivers have to setup supporting ICs to
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do audio/video muxing/encoding/decoding makes it more complex than most.
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Usually these ICs are connected to the main bridge driver through one or
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more I2C busses, but other busses can also be used. Such devices are
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called 'sub-devices'.
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For a long time the framework was limited to the video_device struct for
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creating V4L device nodes and video_buf for handling the video buffers
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(note that this document does not discuss the video_buf framework).
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This meant that all drivers had to do the setup of device instances and
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connecting to sub-devices themselves. Some of this is quite complicated
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to do right and many drivers never did do it correctly.
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There is also a lot of common code that could never be refactored due to
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the lack of a framework.
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So this framework sets up the basic building blocks that all drivers
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need and this same framework should make it much easier to refactor
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common code into utility functions shared by all drivers.
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Structure of a driver
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---------------------
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All drivers have the following structure:
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1) A struct for each device instance containing the device state.
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2) A way of initializing and commanding sub-devices (if any).
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3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX, /dev/radioX and
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/dev/vtxX) and keeping track of device-node specific data.
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4) Filehandle-specific structs containing per-filehandle data.
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This is a rough schematic of how it all relates:
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device instances
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+-sub-device instances
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\-V4L2 device nodes
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\-filehandle instances
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Structure of the framework
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--------------------------
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The framework closely resembles the driver structure: it has a v4l2_device
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struct for the device instance data, a v4l2_subdev struct to refer to
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sub-device instances, the video_device struct stores V4L2 device node data
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and in the future a v4l2_fh struct will keep track of filehandle instances
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(this is not yet implemented).
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struct v4l2_device
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------------------
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Each device instance is represented by a struct v4l2_device (v4l2-device.h).
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Very simple devices can just allocate this struct, but most of the time you
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would embed this struct inside a larger struct.
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You must register the device instance:
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v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
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Registration will initialize the v4l2_device struct and link dev->driver_data
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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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Unregistering will also automatically unregister all subdevs from the device.
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Sometimes you need to iterate over all devices registered by a specific
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driver. This is usually the case if multiple device drivers use the same
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hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
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hardware. The same is true for alsa drivers for example.
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You can iterate over all registered devices as follows:
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static int callback(struct device *dev, void *p)
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{
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struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
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/* test if this device was inited */
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if (v4l2_dev == NULL)
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return 0;
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...
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return 0;
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}
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int iterate(void *p)
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{
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struct device_driver *drv;
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int err;
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/* Find driver 'ivtv' on the PCI bus.
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pci_bus_type is a global. For USB busses use usb_bus_type. */
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drv = driver_find("ivtv", &pci_bus_type);
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/* iterate over all ivtv device instances */
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err = driver_for_each_device(drv, NULL, p, callback);
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put_driver(drv);
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return err;
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}
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Sometimes you need to keep a running counter of the device instance. This is
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commonly used to map a device instance to an index of a module option array.
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The recommended approach is as follows:
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static atomic_t drv_instance = ATOMIC_INIT(0);
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static int __devinit drv_probe(struct pci_dev *dev,
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const struct pci_device_id *pci_id)
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{
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...
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state->instance = atomic_inc_return(&drv_instance) - 1;
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}
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struct v4l2_subdev
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------------------
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Many drivers need to communicate with sub-devices. These devices can do all
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sort of tasks, but most commonly they handle audio and/or video muxing,
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encoding or decoding. For webcams common sub-devices are sensors and camera
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controllers.
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Usually these are I2C devices, but not necessarily. In order to provide the
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driver with a consistent interface to these sub-devices the v4l2_subdev struct
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(v4l2-subdev.h) was created.
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Each sub-device driver must have a v4l2_subdev struct. This struct can be
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stand-alone for simple sub-devices or it might be embedded in a larger struct
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if more state information needs to be stored. Usually there is a low-level
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device struct (e.g. i2c_client) that contains the device data as setup
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by the kernel. It is recommended to store that pointer in the private
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data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
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from a v4l2_subdev to the actual low-level bus-specific device data.
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You also need a way to go from the low-level struct to v4l2_subdev. For the
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common i2c_client struct the i2c_set_clientdata() call is used to store a
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v4l2_subdev pointer, for other busses you may have to use other methods.
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From the bridge driver perspective you load the sub-device module and somehow
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obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
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i2c_get_clientdata(). For other busses something similar needs to be done.
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Helper functions exists for sub-devices on an I2C bus that do most of this
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tricky work for you.
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Each v4l2_subdev contains function pointers that sub-device drivers can
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implement (or leave NULL if it is not applicable). Since sub-devices can do
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so many different things and you do not want to end up with a huge ops struct
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of which only a handful of ops are commonly implemented, the function pointers
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are sorted according to category and each category has its own ops struct.
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The top-level ops struct contains pointers to the category ops structs, which
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may be NULL if the subdev driver does not support anything from that category.
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It looks like this:
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struct v4l2_subdev_core_ops {
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int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
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int (*log_status)(struct v4l2_subdev *sd);
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int (*init)(struct v4l2_subdev *sd, u32 val);
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...
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};
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struct v4l2_subdev_tuner_ops {
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...
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};
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struct v4l2_subdev_audio_ops {
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...
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};
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struct v4l2_subdev_video_ops {
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...
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};
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struct v4l2_subdev_ops {
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const struct v4l2_subdev_core_ops *core;
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const struct v4l2_subdev_tuner_ops *tuner;
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const struct v4l2_subdev_audio_ops *audio;
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const struct v4l2_subdev_video_ops *video;
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};
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The core ops are common to all subdevs, the other categories are implemented
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depending on the sub-device. E.g. a video device is unlikely to support the
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audio ops and vice versa.
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This setup limits the number of function pointers while still making it easy
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to add new ops and categories.
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A sub-device driver initializes the v4l2_subdev struct using:
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v4l2_subdev_init(subdev, &ops);
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Afterwards you need to initialize subdev->name with a unique name and set the
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module owner. This is done for you if you use the i2c helper functions.
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A device (bridge) driver needs to register the v4l2_subdev with the
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v4l2_device:
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int err = v4l2_device_register_subdev(device, subdev);
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This can fail if the subdev module disappeared before it could be registered.
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After this function was called successfully the subdev->dev field points to
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the v4l2_device.
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You can unregister a sub-device using:
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v4l2_device_unregister_subdev(subdev);
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Afterwards the subdev module can be unloaded and subdev->dev == NULL.
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You can call an ops function either directly:
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err = subdev->ops->core->g_chip_ident(subdev, &chip);
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but it is better and easier to use this macro:
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err = v4l2_subdev_call(subdev, core, g_chip_ident, &chip);
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The macro will to the right NULL pointer checks and returns -ENODEV if subdev
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is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
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NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
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It is also possible to call all or a subset of the sub-devices:
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v4l2_device_call_all(dev, 0, core, g_chip_ident, &chip);
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Any subdev that does not support this ops is skipped and error results are
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ignored. If you want to check for errors use this:
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err = v4l2_device_call_until_err(dev, 0, core, g_chip_ident, &chip);
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Any error except -ENOIOCTLCMD will exit the loop with that error. If no
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errors (except -ENOIOCTLCMD) occured, then 0 is returned.
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The second argument to both calls is a group ID. If 0, then all subdevs are
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called. If non-zero, then only those whose group ID match that value will
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be called. Before a bridge driver registers a subdev it can set subdev->grp_id
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to whatever value it wants (it's 0 by default). This value is owned by the
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bridge driver and the sub-device driver will never modify or use it.
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The group ID gives the bridge driver more control how callbacks are called.
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For example, there may be multiple audio chips on a board, each capable of
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changing the volume. But usually only one will actually be used when the
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user want to change the volume. You can set the group ID for that subdev to
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e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
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v4l2_device_call_all(). That ensures that it will only go to the subdev
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that needs it.
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The advantage of using v4l2_subdev is that it is a generic struct and does
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not contain any knowledge about the underlying hardware. So a driver might
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contain several subdevs that use an I2C bus, but also a subdev that is
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controlled through GPIO pins. This distinction is only relevant when setting
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up the device, but once the subdev is registered it is completely transparent.
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I2C sub-device drivers
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----------------------
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Since these drivers are so common, special helper functions are available to
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ease the use of these drivers (v4l2-common.h).
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The recommended method of adding v4l2_subdev support to an I2C driver is to
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embed the v4l2_subdev struct into the state struct that is created for each
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I2C device instance. Very simple devices have no state struct and in that case
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you can just create a v4l2_subdev directly.
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A typical state struct would look like this (where 'chipname' is replaced by
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the name of the chip):
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struct chipname_state {
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struct v4l2_subdev sd;
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... /* additional state fields */
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};
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Initialize the v4l2_subdev struct as follows:
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v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
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This function will fill in all the fields of v4l2_subdev and ensure that the
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v4l2_subdev and i2c_client both point to one another.
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You should also add a helper inline function to go from a v4l2_subdev pointer
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to a chipname_state struct:
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static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
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{
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return container_of(sd, struct chipname_state, sd);
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}
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Use this to go from the v4l2_subdev struct to the i2c_client struct:
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struct i2c_client *client = v4l2_get_subdevdata(sd);
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And this to go from an i2c_client to a v4l2_subdev struct:
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struct v4l2_subdev *sd = i2c_get_clientdata(client);
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Finally you need to make a command function to make driver->command()
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call the right subdev_ops functions:
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static int subdev_command(struct i2c_client *client, unsigned cmd, void *arg)
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{
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return v4l2_subdev_command(i2c_get_clientdata(client), cmd, arg);
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}
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If driver->command is never used then you can leave this out. Eventually the
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driver->command usage should be removed from v4l.
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Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
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is called. This will unregister the sub-device from the bridge driver. It is
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safe to call this even if the sub-device was never registered.
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You need to do this because when the bridge driver destroys the i2c adapter
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the remove() callbacks are called of the i2c devices on that adapter.
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After that the corresponding v4l2_subdev structures are invalid, so they
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have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
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from the remove() callback ensures that this is always done correctly.
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The bridge driver also has some helper functions it can use:
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struct v4l2_subdev *sd = v4l2_i2c_new_subdev(adapter, "module_foo", "chipid", 0x36);
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This loads the given module (can be NULL if no module needs to be loaded) and
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calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
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If all goes well, then it registers the subdev with the v4l2_device. It gets
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the v4l2_device by calling i2c_get_adapdata(adapter), so you should make sure
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that adapdata is set to v4l2_device when you setup the i2c_adapter in your
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driver.
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You can also use v4l2_i2c_new_probed_subdev() which is very similar to
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v4l2_i2c_new_subdev(), except that it has an array of possible I2C addresses
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that it should probe. Internally it calls i2c_new_probed_device().
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Both functions return NULL if something went wrong.
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struct video_device
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-------------------
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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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- v4l2_dev: set to the v4l2_device parent device.
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- name: set to something descriptive and unique.
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- fops: set to the v4l2_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 either .unlocked_ioctl or
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.ioctl to video_ioctl2 in your v4l2_file_operations struct.
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The v4l2_file_operations struct is a subset of file_operations. The main
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difference is that the inode argument is omitted since it is never used.
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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 = vdev->v4l2_dev;
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