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Diffstat (limited to 'Documentation/driver-api/media/camera-sensor.rst')
| -rw-r--r-- | Documentation/driver-api/media/camera-sensor.rst | 24 |
1 files changed, 17 insertions, 7 deletions
diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst index c290833165e6..94bd1dae82d5 100644 --- a/Documentation/driver-api/media/camera-sensor.rst +++ b/Documentation/driver-api/media/camera-sensor.rst @@ -29,21 +29,31 @@ used in the system. Using another frequency may cause harmful effects elsewhere. Therefore only the pre-determined frequencies are configurable by the user. +The external clock frequency shall be retrieved by obtaining the external clock +using the ``devm_v4l2_sensor_clk_get()`` helper function, and then getting its +frequency with ``clk_get_rate()``. Usage of the helper function guarantees +correct behaviour regardless of whether the sensor is integrated in a DT-based +or ACPI-based system. + ACPI ~~~~ -Read the ``clock-frequency`` _DSD property to denote the frequency. The driver -can rely on this frequency being used. +ACPI-based systems typically don't register the sensor external clock with the +kernel, but specify the external clock frequency in the ``clock-frequency`` +_DSD property. The ``devm_v4l2_sensor_clk_get()`` helper creates and returns a +fixed clock set at that rate. Devicetree ~~~~~~~~~~ -The preferred way to achieve this is using ``assigned-clocks``, -``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See the -`clock device tree bindings +Devicetree-based systems declare the sensor external clock in the device tree +and reference it from the sensor node. The preferred way to select the external +clock frequency is to use the ``assigned-clocks``, ``assigned-clock-parents`` +and ``assigned-clock-rates`` properties in the sensor node to set the clock +rate. See the `clock device tree bindings <https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/clock/clock.yaml>`_ -for more information. The driver then gets the frequency using -``clk_get_rate()``. +for more information. The ``devm_v4l2_sensor_clk_get()`` helper retrieves and +returns that clock. This approach has the drawback that there's no guarantee that the frequency hasn't been modified directly or indirectly by another driver, or supported by |