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Commit 378d4a54 authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab
Browse files

[media] v4l2-fh.rst: add fh contents from v4l2-framework.rst 



Move the v4l2_fh specific content from v4l2-framework.rst
to v4l2-fh.rst.

Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@s-opensource.com>
parent 4ada120e

Documentation/media/kapi/v4l2-fh.rst

+134 −0
Original line number Diff line number Diff line 
V4L2 File handlers

------------------



struct v4l2_fh provides a way to easily keep file handle specific data

that is used by the V4L2 framework. New drivers must use struct v4l2_fh

since it is also used to implement priority handling (VIDIOC_G/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 v4l2_fh_init+v4l2_fh_add in open() and

v4l2_fh_del+v4l2_fh_exit in release().



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

macro. Example:



.. code-block:: none



	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:



.. code-block:: none



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



  Initialise the file handle. This *MUST* be performed in the driver's

  v4l2_file_operations->open() handler.



.. code-block:: none



	void v4l2_fh_add(struct v4l2_fh *fh)



  Add a v4l2_fh to video_device file handle list. Must be called once the

  file handle is completely initialized.



.. code-block:: none



	void v4l2_fh_del(struct v4l2_fh *fh)



  Unassociate the file handle from video_device(). The file handle

  exit function may now be called.



.. code-block:: none



	void v4l2_fh_exit(struct v4l2_fh *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:



.. code-block:: none



	int 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.



.. code-block:: none



	int 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:



.. code-block:: none



	int v4l2_fh_is_singular(struct v4l2_fh *fh)



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



.. code-block:: none



	int v4l2_fh_is_singular_file(struct file *filp)



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





V4L2 File Handler kAPI

^^^^^^^^^^^^^^^^^^^^^^

.. kernel-doc:: include/media/v4l2-fh.h

Documentation/media/kapi/v4l2-framework.rst

+0 −133
Original line number Diff line number Diff line
@@ -80,139 +80,6 @@ The V4L2 framework also optionally integrates with the media framework. If a 
driver sets the struct v4l2_device mdev field, sub-devices and video nodes

will automatically appear in the media framework as entities.



struct v4l2_fh

--------------



struct v4l2_fh provides a way to easily keep file handle specific data

that is used by the V4L2 framework. New drivers must use struct v4l2_fh

since it is also used to implement priority handling (VIDIOC_G/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 v4l2_fh_init+v4l2_fh_add in open() and

v4l2_fh_del+v4l2_fh_exit in release().



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

macro. Example:



.. code-block:: none



	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:



.. code-block:: none



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



  Initialise the file handle. This *MUST* be performed in the driver's

  v4l2_file_operations->open() handler.



.. code-block:: none



	void v4l2_fh_add(struct v4l2_fh *fh)



  Add a v4l2_fh to video_device file handle list. Must be called once the

  file handle is completely initialized.



.. code-block:: none



	void v4l2_fh_del(struct v4l2_fh *fh)



  Unassociate the file handle from video_device(). The file handle

  exit function may now be called.



.. code-block:: none



	void v4l2_fh_exit(struct v4l2_fh *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:



.. code-block:: none



	int 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.



.. code-block:: none



	int 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:



.. code-block:: none



	int v4l2_fh_is_singular(struct v4l2_fh *fh)



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



.. code-block:: none



	int v4l2_fh_is_singular_file(struct file *filp)



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



V4L2 clocks

-----------