ftrace: Synchronize the initialization of ftrace_opsgrafted

There's some internal state that ftrace_ops needs to have set, but since
it can be declared outside of the ftrace.c code, it calls
ftrace_ops_init() on the ops in every global function. The issue is that
if two tasks call it on the same ops at the same time it is possible to
have the initialization of one corrupt the initialization of the other
call.

Create a ops_mutex to use to synchronize every initialization of the
ftrace_ops. The mutex is taken within checking the ftrace_ops flag that
states it was initializied but the flag is checked again after the mutex
has been taken. Checking first outside the mutex allows it to shortcut
having to take the mutex. But then the check needs to be done again after
the mute is taken in case of races.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260902095501.6b59af20@gandalf.local.home
Fixes: f04f24fb7e48d ("ftrace, kprobes: Fix a deadlock on ftrace_regex_lock")
Reported-by: sashiko-bot@kernel.org
Close: https://lore.kernel.org/all/20260829025528.49A831F000E9@smtp.kernel.org/
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 145 insertions, 0 deletions

diff --git a/Documentation/driver-api/media/tx-rx.rst b/Documentation/driver-api/media/tx-rx.rst
new file mode 100644
index 000000000..7df240781
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+++ b/Documentation/driver-api/media/tx-rx.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+.. _transmitter-receiver:
+
+Pixel data transmitter and receiver drivers
+===========================================
+
+V4L2 supports various devices that transmit and receive pixel data. Examples of
+these devices include a camera sensor, a TV tuner and a parallel, a BT.656 or a
+CSI-2 receiver in an SoC.
+
+Bus types
+---------
+
+The following buses are the most common. This section discusses these two only.
+
+MIPI CSI-2
+^^^^^^^^^^
+
+CSI-2 is a data bus intended for transferring images from cameras to
+the host SoC. It is defined by the `MIPI alliance`_.
+
+.. _`MIPI alliance`: https://www.mipi.org/
+
+Parallel and BT.656
+^^^^^^^^^^^^^^^^^^^
+
+The parallel and `BT.656`_ buses transport one bit of data on each clock cycle
+per data line. The parallel bus uses synchronisation and other additional
+signals whereas BT.656 embeds synchronisation.
+
+.. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656
+
+Transmitter drivers
+-------------------
+
+Transmitter drivers generally need to provide the receiver drivers with the
+configuration of the transmitter. What is required depends on the type of the
+bus. These are common for both buses.
+
+Media bus pixel code
+^^^^^^^^^^^^^^^^^^^^
+
+See :ref:`v4l2-mbus-pixelcode`.
+
+Link frequency
+^^^^^^^^^^^^^^
+
+The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` control is used to tell the
+receiver the frequency of the bus (i.e. it is not the same as the symbol rate).
+
+Drivers that do not have user-configurable link frequency should report it
+through the ``.get_mbus_config()`` subdev pad operation, in the ``link_freq``
+field of struct v4l2_mbus_config, instead of through controls.
+
+Receiver drivers should use :c:func:`v4l2_get_link_freq` helper to obtain the
+link frequency from the transmitter sub-device.
+
+``.enable_streams()`` and ``.disable_streams()`` callbacks
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The struct v4l2_subdev_pad_ops->enable_streams() and struct
+v4l2_subdev_pad_ops->disable_streams() callbacks are used by the receiver driver
+to control the transmitter driver's streaming state. These callbacks may not be
+called directly, but by using ``v4l2_subdev_enable_streams()`` and
+``v4l2_subdev_disable_streams()``.
+
+Stopping the transmitter
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+A transmitter stops sending the stream of images as a result of
+calling the ``.disable_streams()`` callback. Some transmitters may stop the
+stream at a frame boundary whereas others stop immediately,
+effectively leaving the current frame unfinished. The receiver driver
+should not make assumptions either way, but function properly in both
+cases.
+
+CSI-2 transmitter drivers
+-------------------------
+
+Pixel rate
+^^^^^^^^^^
+
+The pixel rate on the bus is calculated as follows::
+
+	pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample
+
+where
+
+.. list-table:: variables in pixel rate calculation
+   :header-rows: 1
+
+   * - variable or constant
+     - description
+   * - link_freq
+     - The value of the :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` integer64
+       menu item.
+   * - nr_of_lanes
+     - Number of data lanes used on the CSI-2 link.
+   * - 2
+     - Data is transferred on both rising and falling edge of the signal.
+   * - bits_per_sample
+     - Number of bits per sample.
+   * - k
+     - 16 for D-PHY and 7 for C-PHY.
+
+Information on whether D-PHY or C-PHY is used, and the value of ``nr_of_lanes``, can be obtained from the OF endpoint configuration.
+
+.. note::
+
+	The pixel rate calculated this way is **not** the same thing as the
+	pixel rate on the camera sensor's pixel array which is indicated by the
+	:ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control.
+
+LP-11 and LP-111 states
+^^^^^^^^^^^^^^^^^^^^^^^
+
+As part of transitioning to high speed mode, a CSI-2 transmitter typically
+briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period
+may be as short as 100 µs, during which the receiver observes this state and
+proceeds its own part of high speed mode transition.
+
+Most receivers are capable of autonomously handling this once the software has
+configured them to do so, but there are receivers which require software
+involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit
+in software, especially when there is no interrupt telling something is
+happening.
+
+One way to address this is to configure the transmitter side explicitly to LP-11
+or LP-111 state, which requires support from the transmitter hardware. This is
+not universally available. Many devices return to this state once streaming is
+stopped while the state after power-on is LP-00 or LP-000.
+
+The ``.pre_streamon()`` callback may be used to prepare a transmitter for
+transitioning to streaming state, but not yet start streaming. Similarly, the
+``.post_streamoff()`` callback is used to undo what was done by the
+``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required
+to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``.
+
+In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition
+the transmitter to the LP-11 or LP-111 state. This also requires powering on the
+device, so this should be only done when it is needed.
+
+Receiver drivers that do not need explicit LP-11 or LP-111 state setup are
+waived from calling the two callbacks.