140 lines
3.7 KiB
ReStructuredText
140 lines
3.7 KiB
ReStructuredText
V4L2 File handlers
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------------------
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struct :c:type:`v4l2_fh` provides a way to easily keep file handle specific
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data that is used by the V4L2 framework.
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.. attention::
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New drivers must use struct :c:type:`v4l2_fh`
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since it is also used to implement priority handling
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(:ref:`VIDIOC_G_PRIORITY`).
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The users of :c:type:`v4l2_fh` (in the V4L2 framework, not the driver) know
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whether a driver uses :c:type:`v4l2_fh` as its ``file->private_data`` pointer
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by testing the ``V4L2_FL_USES_V4L2_FH`` bit in :c:type:`video_device`->flags.
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This bit is set whenever :c:func:`v4l2_fh_init` is called.
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struct :c:type:`v4l2_fh` is allocated as a part of the driver's own file handle
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structure and ``file->private_data`` is set to it in the driver's ``open()``
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function by the driver.
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In many cases the struct :c:type:`v4l2_fh` will be embedded in a larger
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structure. In that case you should call:
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#) :c:func:`v4l2_fh_init` and :c:func:`v4l2_fh_add` in ``open()``
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#) :c:func:`v4l2_fh_del` and :c:func:`v4l2_fh_exit` in ``release()``
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Drivers can extract their own file handle structure by using the container_of
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macro.
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Example:
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.. code-block:: c
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struct my_fh {
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int blah;
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struct v4l2_fh fh;
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};
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...
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int my_open(struct file *file)
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{
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struct my_fh *my_fh;
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struct video_device *vfd;
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int ret;
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...
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my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
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...
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v4l2_fh_init(&my_fh->fh, vfd);
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...
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file->private_data = &my_fh->fh;
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v4l2_fh_add(&my_fh->fh);
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return 0;
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}
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int my_release(struct file *file)
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{
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struct v4l2_fh *fh = file->private_data;
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struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
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...
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v4l2_fh_del(&my_fh->fh);
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v4l2_fh_exit(&my_fh->fh);
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kfree(my_fh);
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return 0;
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}
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Below is a short description of the :c:type:`v4l2_fh` functions used:
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:c:func:`v4l2_fh_init <v4l2_fh_init>`
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(:c:type:`fh <v4l2_fh>`, :c:type:`vdev <video_device>`)
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- Initialise the file handle. This **MUST** be performed in the driver's
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:c:type:`v4l2_file_operations`->open() handler.
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:c:func:`v4l2_fh_add <v4l2_fh_add>`
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(:c:type:`fh <v4l2_fh>`)
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- Add a :c:type:`v4l2_fh` to :c:type:`video_device` file handle list.
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Must be called once the file handle is completely initialized.
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:c:func:`v4l2_fh_del <v4l2_fh_del>`
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(:c:type:`fh <v4l2_fh>`)
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- Unassociate the file handle from :c:type:`video_device`. The file handle
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exit function may now be called.
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:c:func:`v4l2_fh_exit <v4l2_fh_exit>`
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(:c:type:`fh <v4l2_fh>`)
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- Uninitialise the file handle. After uninitialisation the :c:type:`v4l2_fh`
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memory can be freed.
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If struct :c:type:`v4l2_fh` is not embedded, then you can use these helper functions:
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:c:func:`v4l2_fh_open <v4l2_fh_open>`
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(struct file \*filp)
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- This allocates a struct :c:type:`v4l2_fh`, initializes it and adds it to
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the struct :c:type:`video_device` associated with the file struct.
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:c:func:`v4l2_fh_release <v4l2_fh_release>`
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(struct file \*filp)
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- This deletes it from the struct :c:type:`video_device` associated with the
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file struct, uninitialised the :c:type:`v4l2_fh` and frees it.
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These two functions can be plugged into the v4l2_file_operation's ``open()``
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and ``release()`` ops.
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Several drivers need to do something when the first file handle is opened and
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when the last file handle closes. Two helper functions were added to check
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whether the :c:type:`v4l2_fh` struct is the only open filehandle of the
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associated device node:
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:c:func:`v4l2_fh_is_singular <v4l2_fh_is_singular>`
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(:c:type:`fh <v4l2_fh>`)
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- Returns 1 if the file handle is the only open file handle, else 0.
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:c:func:`v4l2_fh_is_singular_file <v4l2_fh_is_singular_file>`
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(struct file \*filp)
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- Same, but it calls v4l2_fh_is_singular with filp->private_data.
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V4L2 fh functions and data structures
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. kernel-doc:: include/media/v4l2-fh.h
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