| 1 | .. SPDX-License-Identifier: GPL-2.0 |
| 2 | |
| 3 | .. _media_writing_camera_sensor_drivers: |
| 4 | |
| 5 | Writing camera sensor drivers |
| 6 | ============================= |
| 7 | |
| 8 | This document covers the in-kernel APIs only. For the best practices on |
| 9 | userspace API implementation in camera sensor drivers, please see |
| 10 | :ref:`media_using_camera_sensor_drivers`. |
| 11 | |
| 12 | CSI-2, parallel and BT.656 buses |
| 13 | -------------------------------- |
| 14 | |
| 15 | Please see :ref:`transmitter-receiver`. |
| 16 | |
| 17 | Handling clocks |
| 18 | --------------- |
| 19 | |
| 20 | Camera sensors have an internal clock tree including a PLL and a number of |
| 21 | divisors. The clock tree is generally configured by the driver based on a few |
| 22 | input parameters that are specific to the hardware: the external clock frequency |
| 23 | and the link frequency. The two parameters generally are obtained from system |
| 24 | firmware. **No other frequencies should be used in any circumstances.** |
| 25 | |
| 26 | The reason why the clock frequencies are so important is that the clock signals |
| 27 | come out of the SoC, and in many cases a specific frequency is designed to be |
| 28 | used in the system. Using another frequency may cause harmful effects |
| 29 | elsewhere. Therefore only the pre-determined frequencies are configurable by the |
| 30 | user. |
| 31 | |
| 32 | ACPI |
| 33 | ~~~~ |
| 34 | |
| 35 | Read the ``clock-frequency`` _DSD property to denote the frequency. The driver |
| 36 | can rely on this frequency being used. |
| 37 | |
| 38 | Devicetree |
| 39 | ~~~~~~~~~~ |
| 40 | |
| 41 | The preferred way to achieve this is using ``assigned-clocks``, |
| 42 | ``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See the |
| 43 | `clock device tree bindings |
| 44 | <https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/clock/clock.yaml>`_ |
| 45 | for more information. The driver then gets the frequency using |
| 46 | ``clk_get_rate()``. |
| 47 | |
| 48 | This approach has the drawback that there's no guarantee that the frequency |
| 49 | hasn't been modified directly or indirectly by another driver, or supported by |
| 50 | the board's clock tree to begin with. Changes to the Common Clock Framework API |
| 51 | are required to ensure reliability. |
| 52 | |
| 53 | Power management |
| 54 | ---------------- |
| 55 | |
| 56 | Camera sensors are used in conjunction with other devices to form a camera |
| 57 | pipeline. They must obey the rules listed herein to ensure coherent power |
| 58 | management over the pipeline. |
| 59 | |
| 60 | Camera sensor drivers are responsible for controlling the power state of the |
| 61 | device they otherwise control as well. They shall use runtime PM to manage |
| 62 | power states. Runtime PM shall be enabled at probe time and disabled at remove |
| 63 | time. Drivers should enable runtime PM autosuspend. Also see |
| 64 | :ref:`async sub-device registration <media-registering-async-subdevs>`. |
| 65 | |
| 66 | The runtime PM handlers shall handle clocks, regulators, GPIOs, and other |
| 67 | system resources required to power the sensor up and down. For drivers that |
| 68 | don't use any of those resources (such as drivers that support ACPI systems |
| 69 | only), the runtime PM handlers may be left unimplemented. |
| 70 | |
| 71 | In general, the device shall be powered on at least when its registers are |
| 72 | being accessed and when it is streaming. Drivers should use |
| 73 | ``pm_runtime_resume_and_get()`` when starting streaming and |
| 74 | ``pm_runtime_put()`` or ``pm_runtime_put_autosuspend()`` when stopping |
| 75 | streaming. They may power the device up at probe time (for example to read |
| 76 | identification registers), but should not keep it powered unconditionally after |
| 77 | probe. |
| 78 | |
| 79 | At system suspend time, the whole camera pipeline must stop streaming, and |
| 80 | restart when the system is resumed. This requires coordination between the |
| 81 | camera sensor and the rest of the camera pipeline. Bridge drivers are |
| 82 | responsible for this coordination, and instruct camera sensors to stop and |
| 83 | restart streaming by calling the appropriate subdev operations |
| 84 | (``.s_stream()``, ``.enable_streams()`` or ``.disable_streams()``). Camera |
| 85 | sensor drivers shall therefore **not** keep track of the streaming state to |
| 86 | stop streaming in the PM suspend handler and restart it in the resume handler. |
| 87 | Drivers should in general not implement the system PM handlers. |
| 88 | |
| 89 | Camera sensor drivers shall **not** implement the subdev ``.s_power()`` |
| 90 | operation, as it is deprecated. While this operation is implemented in some |
| 91 | existing drivers as they predate the deprecation, new drivers shall use runtime |
| 92 | PM instead. If you feel you need to begin calling ``.s_power()`` from an ISP or |
| 93 | a bridge driver, instead add runtime PM support to the sensor driver you are |
| 94 | using and drop its ``.s_power()`` handler. |
| 95 | |
| 96 | Please also see :ref:`examples <media-camera-sensor-examples>`. |
| 97 | |
| 98 | Control framework |
| 99 | ~~~~~~~~~~~~~~~~~ |
| 100 | |
| 101 | ``v4l2_ctrl_handler_setup()`` function may not be used in the device's runtime |
| 102 | PM ``runtime_resume`` callback, as it has no way to figure out the power state |
| 103 | of the device. This is because the power state of the device is only changed |
| 104 | after the power state transition has taken place. The ``s_ctrl`` callback can be |
| 105 | used to obtain device's power state after the power state transition: |
| 106 | |
| 107 | .. c:function:: int pm_runtime_get_if_in_use(struct device *dev); |
| 108 | |
| 109 | The function returns a non-zero value if it succeeded getting the power count or |
| 110 | runtime PM was disabled, in either of which cases the driver may proceed to |
| 111 | access the device. |
| 112 | |
| 113 | Rotation, orientation and flipping |
| 114 | ---------------------------------- |
| 115 | |
| 116 | Use ``v4l2_fwnode_device_parse()`` to obtain rotation and orientation |
| 117 | information from system firmware and ``v4l2_ctrl_new_fwnode_properties()`` to |
| 118 | register the appropriate controls. |
| 119 | |
| 120 | .. _media-camera-sensor-examples: |
| 121 | |
| 122 | Example drivers |
| 123 | --------------- |
| 124 | |
| 125 | Features implemented by sensor drivers vary, and depending on the set of |
| 126 | supported features and other qualities, particular sensor drivers better serve |
| 127 | the purpose of an example. The following drivers are known to be good examples: |
| 128 | |
| 129 | .. flat-table:: Example sensor drivers |
| 130 | :header-rows: 0 |
| 131 | :widths: 1 1 1 2 |
| 132 | |
| 133 | * - Driver name |
| 134 | - File(s) |
| 135 | - Driver type |
| 136 | - Example topic |
| 137 | * - CCS |
| 138 | - ``drivers/media/i2c/ccs/`` |
| 139 | - Freely configurable |
| 140 | - Power management (ACPI and DT), UAPI |
| 141 | * - imx219 |
| 142 | - ``drivers/media/i2c/imx219.c`` |
| 143 | - Register list based |
| 144 | - Power management (DT), UAPI, mode selection |
| 145 | * - imx319 |
| 146 | - ``drivers/media/i2c/imx319.c`` |
| 147 | - Register list based |
| 148 | - Power management (ACPI and DT) |