1.5. V4L2 device instance¶

Each device instance is represented by a struct v4l2_device. Very simple devices can just allocate this struct, but most of the time you would embed this struct inside a larger struct.

You must register the device instance by calling:

v4l2_device_register (dev, v4l2_dev).

Registration will initialize the v4l2_device struct. If the dev->driver_data field is NULL, it will be linked to v4l2_dev argument.

Drivers that want integration with the media device framework need to set dev->driver_data manually to point to the driver-specific device structure that embed the struct v4l2_device instance. This is achieved by a dev_set_drvdata() call before registering the V4L2 device instance. They must also set the struct v4l2_device mdev field to point to a properly initialized and registered media_device instance.

If v4l2_dev->name is empty then it will be set to a value derived from dev (driver name followed by the bus_id, to be precise). If you set it up before calling v4l2_device_register() then it will be untouched. If dev is NULL, then you must setup v4l2_dev->name before calling v4l2_device_register().

You can use v4l2_device_set_name() to set the name based on a driver name and a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1, etc. If the name ends with a digit, then it will insert a dash: cx18-0, cx18-1, etc. This function returns the instance number.

The first dev argument is normally the struct device pointer of a pci_dev, usb_interface or platform_device. It is rare for dev to be NULL, but it happens with ISA devices or when one device creates multiple PCI devices, thus making it impossible to associate v4l2_dev with a particular parent.

You can also supply a notify() callback that can be called by sub-devices to notify you of events. Whether you need to set this depends on the sub-device. Any notifications a sub-device supports must be defined in a header in include/media/subdevice.h.

V4L2 devices are unregistered by calling:

v4l2_device_unregister() (v4l2_dev).

If the dev->driver_data field points to v4l2_dev, it will be reset to NULL. Unregistering will also automatically unregister all subdevs from the device.

If you have a hotpluggable device (e.g. a USB device), then when a disconnect happens the parent device becomes invalid. Since v4l2_device has a pointer to that parent device it has to be cleared as well to mark that the parent is gone. To do this call:

v4l2_device_disconnect() (v4l2_dev).

This does not unregister the subdevs, so you still need to call the v4l2_device_unregister() function for that. If your driver is not hotpluggable, then there is no need to call v4l2_device_disconnect().

Sometimes you need to iterate over all devices registered by a specific driver. This is usually the case if multiple device drivers use the same hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv hardware. The same is true for alsa drivers for example.

You can iterate over all registered devices as follows:

static int callback(struct device *dev, void *p)
{
        struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);

        /* test if this device was inited */
        if (v4l2_dev == NULL)
                return 0;
        ...
        return 0;
}

int iterate(void *p)
{
        struct device_driver *drv;
        int err;

        /* Find driver 'ivtv' on the PCI bus.
        pci_bus_type is a global. For USB busses use usb_bus_type. */
        drv = driver_find("ivtv", &pci_bus_type);
        /* iterate over all ivtv device instances */
        err = driver_for_each_device(drv, NULL, p, callback);
        put_driver(drv);
        return err;
}

Sometimes you need to keep a running counter of the device instance. This is commonly used to map a device instance to an index of a module option array.

The recommended approach is as follows:

static atomic_t drv_instance = ATOMIC_INIT(0);

static int drv_probe(struct pci_dev *pdev, const struct pci_device_id *pci_id)
{
        ...
        state->instance = atomic_inc_return(&drv_instance) - 1;
}

If you have multiple device nodes then it can be difficult to know when it is safe to unregister v4l2_device for hotpluggable devices. For this purpose v4l2_device has refcounting support. The refcount is increased whenever video_register_device() is called and it is decreased whenever that device node is released. When the refcount reaches zero, then the v4l2_device release() callback is called. You can do your final cleanup there.

If other device nodes (e.g. ALSA) are created, then you can increase and decrease the refcount manually as well by calling:

v4l2_device_get() (v4l2_dev).

or:

v4l2_device_put() (v4l2_dev).

Since the initial refcount is 1 you also need to call v4l2_device_put() in the disconnect() callback (for USB devices) or in the remove() callback (for e.g. PCI devices), otherwise the refcount will never reach 0.

1.5.1. v4l2_device functions and data structures¶

struct v4l2_device¶: main struct to for V4L2 device drivers

Definition

struct v4l2_device {
  struct device * dev;
#if defined(CONFIG_MEDIA_CONTROLLER
  struct media_device * mdev;
#endif
  struct list_head subdevs;
  spinlock_t lock;
  char name;
  void (* notify) (struct v4l2_subdev *sd, unsigned int notification, void *arg);
  struct v4l2_ctrl_handler * ctrl_handler;
  struct v4l2_prio_state prio;
  struct kref ref;
  void (* release) (struct v4l2_device *v4l2_dev);
};

Members

dev: pointer to struct device.
mdev: pointer to struct media_device
subdevs: used to keep track of the registered subdevs
lock: lock this struct; can be used by the driver as well if this struct is embedded into a larger struct.
name: unique device name, by default the driver name + bus ID
notify: notify callback called by some sub-devices.
ctrl_handler: The control handler. May be NULL.
prio: Device’s priority state
ref: Keep track of the references to this struct.
release: Release function that is called when the ref count goes to 0.

Description

Each instance of a V4L2 device should create the v4l2_device struct, either stand-alone or embedded in a larger struct.

It allows easy access to sub-devices (see v4l2-subdev.h) and provides basic V4L2 device-level support.

Note

dev->driver_data points to this struct.
dev might be NULL if there is no parent device

void v4l2_device_get(struct v4l2_device * v4l2_dev)¶: gets a V4L2 device reference

Parameters

struct v4l2_device * v4l2_dev: pointer to struct v4l2_device

Description

This is an ancillary routine meant to increment the usage for the struct v4l2_device pointed by v4l2_dev.

int v4l2_device_put(struct v4l2_device * v4l2_dev)¶: putss a V4L2 device reference

Parameters

struct v4l2_device * v4l2_dev: pointer to struct v4l2_device

Description

This is an ancillary routine meant to decrement the usage for the struct v4l2_device pointed by v4l2_dev.

int v4l2_device_register(struct device * dev, struct v4l2_device * v4l2_dev)¶: Initialize v4l2_dev and make dev->driver_data point to v4l2_dev.

Parameters

struct device * dev: pointer to struct device
struct v4l2_device * v4l2_dev: pointer to struct v4l2_device

Description

Note

dev may be NULL in rare cases (ISA devices). In such case the caller must fill in the v4l2_dev->name field before calling this function.

int v4l2_device_set_name(struct v4l2_device * v4l2_dev, const char * basename, atomic_t * instance)¶: Optional function to initialize the name field of struct v4l2_device

Parameters

struct v4l2_device * v4l2_dev: pointer to struct v4l2_device
const char * basename: base name for the device name
atomic_t * instance: pointer to a static atomic_t var with the instance usage for the device driver.

Description

v4l2_device_set_name() initializes the name field of struct v4l2_device using the driver name and a driver-global atomic_t instance.

This function will increment the instance counter and returns the instance value used in the name.

Example

static atomic_t drv_instance = ATOMIC_INIT(0);

...

instance = v4l2_device_set_name(&v4l2_dev, “foo”, &drv_instance);

The first time this is called the name field will be set to foo0 and this function returns 0. If the name ends with a digit (e.g. cx18), then the name will be set to cx18-0 since cx180 would look really odd.

void v4l2_device_disconnect(struct v4l2_device * v4l2_dev)¶: Change V4L2 device state to disconnected.

Parameters

struct v4l2_device * v4l2_dev: pointer to struct v4l2_device

Description

Should be called when the USB parent disconnects. Since the parent disappears, this ensures that v4l2_dev doesn’t have an invalid parent pointer.

Note

This function sets v4l2_dev->dev to NULL.

void v4l2_device_unregister(struct v4l2_device * v4l2_dev)¶: Unregister all sub-devices and any other resources related to v4l2_dev.

Parameters

struct v4l2_device * v4l2_dev: pointer to struct v4l2_device

int v4l2_device_register_subdev(struct v4l2_device * v4l2_dev, struct v4l2_subdev * sd)¶: Registers a subdev with a v4l2 device.

Parameters

struct v4l2_device * v4l2_dev: pointer to struct v4l2_device
struct v4l2_subdev * sd: pointer to struct v4l2_subdev

Description

While registered, the subdev module is marked as in-use.

An error is returned if the module is no longer loaded on any attempts to register it.

void v4l2_device_unregister_subdev(struct v4l2_subdev * sd)¶: Unregisters a subdev with a v4l2 device.

Parameters

struct v4l2_subdev * sd: pointer to struct v4l2_subdev

Description

Note

Can also be called if the subdev wasn’t registered. In such case, it will do nothing.

int v4l2_device_register_subdev_nodes(struct v4l2_device * v4l2_dev)¶: Registers device nodes for all subdevs of the v4l2 device that are marked with the V4L2_SUBDEV_FL_HAS_DEVNODE flag.

Parameters

struct v4l2_device * v4l2_dev: pointer to struct v4l2_device

void v4l2_subdev_notify(struct v4l2_subdev * sd, unsigned int notification, void * arg)¶: Sends a notification to v4l2_device.

Parameters

struct v4l2_subdev * sd: pointer to struct v4l2_subdev
unsigned int notification: type of notification. Please notice that the notification type is driver-specific.
void * arg: arguments for the notification. Those are specific to each notification type.