[PATCH v3 1/2] libcamera: v4l2_device: add frame start event helpers

[Kieran Bingham]

Quoting Stanislaw Gruszka (2025-02-18 09:19:50)
> Add helper to check if frame start event are supported by subdevice.
> Since kernel does not have interface to query supported events
> use subscribe interface.
> 
> Additionally add helper for checking if the event enabled.
> Will be needed later to proper code flow if enabling the event
> fail for some reason.
> 

I've started contemplating tackling this too - with a 'V4L2Event'
wrapper ... but that's overkill while we have only a single event to
manage, so I think this is fine for now.

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com>
> ---
>  include/libcamera/internal/v4l2_device.h |  2 ++
>  src/libcamera/v4l2_device.cpp            | 24 ++++++++++++++++++++++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/include/libcamera/internal/v4l2_device.h b/include/libcamera/internal/v4l2_device.h
> index affe52c2..0d807209 100644
> --- a/include/libcamera/internal/v4l2_device.h
> +++ b/include/libcamera/internal/v4l2_device.h
> @@ -45,6 +45,8 @@ public:
>         const std::string &deviceNode() const { return deviceNode_; }
>         std::string devicePath() const;
>  
> +       bool supportsFrameStartEvent();
> +       bool frameStartEnabled() { return frameStartEnabled_; }
>         int setFrameStartEnabled(bool enable);
>         Signal<uint32_t> frameStart;
>  
> diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
> index 2f65a43a..219c82f6 100644
> --- a/src/libcamera/v4l2_device.cpp
> +++ b/src/libcamera/v4l2_device.cpp
> @@ -449,6 +449,30 @@ std::string V4L2Device::devicePath() const
>         return path;
>  }
>  
> +/**
> + * \brief Check if frame start event is supported
> + * \return True if frame start event is supported, false otherwise
> + */
> +
> +bool V4L2Device::supportsFrameStartEvent()
> +{
> +       struct v4l2_event_subscription event {
> +       };
> +       event.type = V4L2_EVENT_FRAME_SYNC;
> +
> +       int ret = ioctl(VIDIOC_SUBSCRIBE_EVENT, &event);
> +       if (ret == 0)
> +               ioctl(VIDIOC_UNSUBSCRIBE_EVENT, &event);
> +
> +       return ret == 0;
> +}
> +
> +/**
> + * \fn V4L2Device::frameStartEnabled()
> + * \brief Check if frame start event is enabled
> + * \return True if frame start event is enabled, false otherwise
> + */
> +
>  /**
>   * \brief Enable or disable frame start event notification
>   * \param[in] enable True to enable frame start events, false to disable them
> -- 
> 2.43.0
>