[libcamera-devel] [PATCH v3 1/6] libcamera: Document sensor driver requirements

[Laurent Pinchart]

Hi Jacopo,

Thank you for the patch.

On Wed, Dec 30, 2020 at 07:01:15PM +0100, Jacopo Mondi wrote:
> Document the feature an image sensor driver has to provide to be
> fully libcamera-compliant.
> 
> Signed-off-by: Jacopo Mondi <jacopo at jmondi.org>
> ---
>  Documentation/index.rst                      |  1 +
>  Documentation/meson.build                    |  1 +
>  Documentation/sensor_driver_requirements.rst | 61 ++++++++++++++++++++
>  3 files changed, 63 insertions(+)
>  create mode 100644 Documentation/sensor_driver_requirements.rst
> 
> diff --git a/Documentation/index.rst b/Documentation/index.rst
> index c49db18d52bd..285ca7c3e5ae 100644
> --- a/Documentation/index.rst
> +++ b/Documentation/index.rst
> @@ -18,3 +18,4 @@
>     Pipeline Handler Writer's Guide <guides/pipeline-handler>
>     Tracing guide <guides/tracing>
>     Environment variables <environment_variables>
> +   Sensor driver requirements <sensor_driver_requirements>

This reminds me we'll probably need to organize the documents at some
point :-)

> diff --git a/Documentation/meson.build b/Documentation/meson.build
> index b1f5bb52474c..9950465ded6e 100644
> --- a/Documentation/meson.build
> +++ b/Documentation/meson.build
> @@ -57,6 +57,7 @@ if sphinx.found()
>          'guides/pipeline-handler.rst',
>          'guides/tracing.rst',
>          'index.rst',
> +        'sensor_driver_requirements.rst',
>         '../README.rst',
>      ]
>  
> diff --git a/Documentation/sensor_driver_requirements.rst b/Documentation/sensor_driver_requirements.rst
> new file mode 100644
> index 000000000000..89ddd58a5ec6
> --- /dev/null
> +++ b/Documentation/sensor_driver_requirements.rst
> @@ -0,0 +1,61 @@
> +.. SPDX-License-Identifier: CC-BY-SA-4.0
> +
> +.. _sensor-driver-requirements:
> +
> +Sensor Driver Requirements
> +==========================
> +
> +libcamera handles imaging devices in the CameraSensor class and defines
> +through its API a consistent interface towards other library components.
> +
> +The CameraSensor class uses the V4L2 subdev kernel API to interface with the
> +camera sensor through one or multiple sub-devices registered in userspace by

Do you mean registered in kernel space ? Or maybe exposed to userspace ?

> +the sensor driver.
> +
> +In order for libcamera to be fully operational and provide to applications and
> +pipeline handlers all the required information to interface with the camera
> +sensor, a set of mandatory and optional features the driver has to support
> +have been defined.

s/have/has/ ("a set")

> +
> +Mandatory Requirements
> +----------------------
> +
> +The sensor driver is assumed to be fully compliant with the V4L2 specification.
> +
> +The sensor driver shall support the following V4L2 controls:
> +
> +* `V4L2_CID_HBLANK`_
> +* `V4L2_CID_PIXEL_RATE`_
> +
> +.. _V4L2_CID_HBLANK: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/ext-ctrls-image-source.html
> +.. _V4L2_CID_PIXEL_RATE: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/ext-ctrls-image-process.html
> +
> +Both controls are used to compute the sensor output timings.
> +
> +Optional Requirements
> +---------------------
> +
> +The sensor driver should support the following V4L2 controls:
> +
> +* `V4L2_CID_CAMERA_ORIENTATION`_
> +* `V4L2_CID_CAMERA_SENSOR_ROTATION`_
> +
> +.. _V4L2_CID_CAMERA_ORIENTATION: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/ext-ctrls-camera.html
> +.. _V4L2_CID_CAMERA_SENSOR_ROTATION: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/ext-ctrls-image-process.html
> +
> +The controls are used to register the camera location and orientation.
> +
> +The sensor driver should implement support for the V4L2 Selection API,
> +specifically it should implement support for the
> +`VIDIOC_SUBDEV_G_SELECTION`_ ioctl with support for the following selection
> +targets:
> +
> +.. _VIDIOC_SUBDEV_G_SELECTION: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/vidioc-subdev-g-selection.html?highlight=g_selection#c.V4L.VIDIOC_SUBDEV_G_SELECTION
> +
> +* `V4L2_SEL_TGT_CROP_BOUNDS`_ to report the readable pixel array area size
> +* `V4L2_SEL_TGT_CROP_DEFAULT`_ to report the active pixel array area size
> +* `V4L2_SEL_TGT_CROP`_ to report the analogue selection rectangle

I would add "Support for the selectino API is scheduled to become a
mandatory requirement in the near future."

Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

> +
> +.. _V4L2_SEL_TGT_CROP_BOUNDS: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/v4l2-selection-targets.html
> +.. _V4L2_SEL_TGT_CROP_DEFAULT: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/v4l2-selection-targets.html
> +.. _V4L2_SEL_TGT_CROP: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/v4l2-selection-targets.html

-- 
Regards,

Laurent Pinchart