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Documentation: media: camera-sensor: Mention v4l2_devm_sensor_clk_get() for obtaining the clock

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Add the new v4l2 helper devm_v4l2_sensor_clk_get() to Documentation. the
helper works on both DT- and ACPI-based platforms to retrieve a
reference to the clock producer from firmware.

Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mehdi Djait <mehdi.djait@linux.intel.com>
Co-developed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
This commit is contained in:
Mehdi Djait 2025-06-26 15:33:53 +02:00 • committed by Hans Verkuil
parent 2d240b124c
commit e2b1ebd72c
1 changed files with 17 additions and 7 deletions
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24
Documentation/driver-api/media/camera-sensor.rst
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@ -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