137 lines
3.8 KiB
ReStructuredText
137 lines
3.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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V4L2 File handles
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-----------------
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struct v4l2_fh provides a way to easily keep file handle specific data that is
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used by the V4L2 framework. Its usage is mandatory in all drivers.
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struct v4l2_fh is allocated in the driver's ``open()`` file operation handler.
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It is typically embedded in a larger driver-specific structure. The
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:c:type:`v4l2_fh` must be initialized with a call to :c:func:`v4l2_fh_init`,
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and added to the video device with :c:func:`v4l2_fh_add`. This associates the
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:c:type:`v4l2_fh` with the :c:type:`file` by setting ``file->private_data`` to
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point to the :c:type:`v4l2_fh`.
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Similarly, the struct v4l2_fh is freed in the driver's ``release()`` file
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operation handler. It must be removed from the video device with
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:c:func:`v4l2_fh_del` and cleaned up with :c:func:`v4l2_fh_exit` before being
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freed.
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Drivers must not access ``file->private_data`` directly. They can retrieve the
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:c:type:`v4l2_fh` associated with a :c:type:`file` by calling
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:c:func:`file_to_v4l2_fh`. Drivers can extract their own file handle structure
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by using the container_of 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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v4l2_fh_add(&my_fh->fh, file);
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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_to_v4l2_fh(file);
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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, file);
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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>`, struct file \*filp)
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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>`, struct file \*filp)
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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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:c:func:`file_to_v4l2_fh <file_to_v4l2_fh>`
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(struct file \*filp)
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- Retrieve the :c:type:`v4l2_fh` instance associated with a :c:type:`file`.
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If struct 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 v4l2_fh, initializes it and adds it to
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the struct 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 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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