1.6. V4L2 File handlers

struct v4l2_fh provides a way to easily keep file handle specific data that is used by the V4L2 framework.

Attention

New drivers must use struct v4l2_fh since it is also used to implement priority handling (ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY).

The users of v4l2_fh (in the V4L2 framework, not the driver) know whether a driver uses v4l2_fh as its file->private_data pointer by testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is set whenever v4l2_fh_init() is called.

struct v4l2_fh is allocated as a part of the driver’s own file handle structure and file->private_data is set to it in the driver’s open() function by the driver.

In many cases the struct v4l2_fh will be embedded in a larger structure. In that case you should call:

  1. v4l2_fh_init() and v4l2_fh_add() in open()

  2. v4l2_fh_del() and v4l2_fh_exit() in release()

Drivers can extract their own file handle structure by using the container_of macro.

Example:

struct my_fh {
        int blah;
        struct v4l2_fh fh;
};

...

int my_open(struct file *file)
{
        struct my_fh *my_fh;
        struct video_device *vfd;
        int ret;

        ...

        my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);

        ...

        v4l2_fh_init(&my_fh->fh, vfd);

        ...

        file->private_data = &my_fh->fh;
        v4l2_fh_add(&my_fh->fh);
        return 0;
}

int my_release(struct file *file)
{
        struct v4l2_fh *fh = file->private_data;
        struct my_fh *my_fh = container_of(fh, struct my_fh, fh);

        ...
        v4l2_fh_del(&my_fh->fh);
        v4l2_fh_exit(&my_fh->fh);
        kfree(my_fh);
        return 0;
}

Below is a short description of the v4l2_fh functions used:

v4l2_fh_init (fh, vdev)

  • Initialise the file handle. This MUST be performed in the driver’s v4l2_file_operations->open() handler.

v4l2_fh_add (fh)

  • Add a v4l2_fh to video_device file handle list. Must be called once the file handle is completely initialized.

v4l2_fh_del (fh)

  • Unassociate the file handle from video_device. The file handle exit function may now be called.

v4l2_fh_exit (fh)

  • Uninitialise the file handle. After uninitialisation the v4l2_fh memory can be freed.

If struct v4l2_fh is not embedded, then you can use these helper functions:

v4l2_fh_open (struct file *filp)

  • This allocates a struct v4l2_fh, initializes it and adds it to the struct video_device associated with the file struct.

v4l2_fh_release (struct file *filp)

  • This deletes it from the struct video_device associated with the file struct, uninitialised the v4l2_fh and frees it.

These two functions can be plugged into the v4l2_file_operation’s open() and release() ops.

Several drivers need to do something when the first file handle is opened and when the last file handle closes. Two helper functions were added to check whether the v4l2_fh struct is the only open filehandle of the associated device node:

v4l2_fh_is_singular (fh)

  • Returns 1 if the file handle is the only open file handle, else 0.

v4l2_fh_is_singular_file (struct file *filp)

  • Same, but it calls v4l2_fh_is_singular with filp->private_data.

1.6.1. V4L2 fh functions and data structures

struct v4l2_fh

Describes a V4L2 file handler

Definition

struct v4l2_fh {
  struct list_head        list;
  struct video_device     *vdev;
  struct v4l2_ctrl_handler *ctrl_handler;
  enum v4l2_priority      prio;
  wait_queue_head_t wait;
  struct mutex            subscribe_lock;
  struct list_head        subscribed;
  struct list_head        available;
  unsigned int            navailable;
  u32 sequence;
  struct v4l2_m2m_ctx     *m2m_ctx;
};

Members

list

list of file handlers

vdev

pointer to struct video_device

ctrl_handler

pointer to struct v4l2_ctrl_handler

prio

priority of the file handler, as defined by enum v4l2_priority

wait

event’ s wait queue

subscribe_lock

serialise changes to the subscribed list; guarantee that the add and del event callbacks are orderly called

subscribed

list of subscribed events

available

list of events waiting to be dequeued

navailable

number of available events at available list

sequence

event sequence number

m2m_ctx

pointer to struct v4l2_m2m_ctx

void v4l2_fh_init(struct v4l2_fh * fh, struct video_device * vdev)

Initialise the file handle.

Parameters

struct v4l2_fh * fh

pointer to struct v4l2_fh

struct video_device * vdev

pointer to struct video_device

Description

Parts of the V4L2 framework using the file handles should be initialised in this function. Must be called from driver’s v4l2_file_operations->open() handler if the driver uses struct v4l2_fh.

void v4l2_fh_add(struct v4l2_fh * fh)

Add the fh to the list of file handles on a video_device.

Parameters

struct v4l2_fh * fh

pointer to struct v4l2_fh

Description

Note

The fh file handle must be initialised first.

int v4l2_fh_open(struct file * filp)

Ancillary routine that can be used as the open() op of v4l2_file_operations.

Parameters

struct file * filp

pointer to struct file

Description

It allocates a v4l2_fh and inits and adds it to the struct video_device associated with the file pointer.

void v4l2_fh_del(struct v4l2_fh * fh)

Remove file handle from the list of file handles.

Parameters

struct v4l2_fh * fh

pointer to struct v4l2_fh

Description

On error filp->private_data will be NULL, otherwise it will point to the struct v4l2_fh.

Note

Must be called in v4l2_file_operations->release() handler if the driver uses struct v4l2_fh.

void v4l2_fh_exit(struct v4l2_fh * fh)

Release resources related to a file handle.

Parameters

struct v4l2_fh * fh

pointer to struct v4l2_fh

Description

Parts of the V4L2 framework using the v4l2_fh must release their resources here, too.

Note

Must be called in v4l2_file_operations->release() handler if the driver uses struct v4l2_fh.

int v4l2_fh_release(struct file * filp)

Ancillary routine that can be used as the release() op of v4l2_file_operations.

Parameters

struct file * filp

pointer to struct file

Description

It deletes and exits the v4l2_fh associated with the file pointer and frees it. It will do nothing if filp->private_data (the pointer to the v4l2_fh struct) is NULL.

This function always returns 0.

int v4l2_fh_is_singular(struct v4l2_fh * fh)

Returns 1 if this filehandle is the only filehandle opened for the associated video_device.

Parameters

struct v4l2_fh * fh

pointer to struct v4l2_fh

Description

If fh is NULL, then it returns 0.

int v4l2_fh_is_singular_file(struct file * filp)

Returns 1 if this filehandle is the only filehandle opened for the associated video_device.

Parameters

struct file * filp

pointer to struct file

Description

This is a helper function variant of v4l2_fh_is_singular() with uses struct file as argument.

If filp->private_data is NULL, then it will return 0.