[PATCH v7 1/5] libcamera: v4l2_device: add frame start event helpers

[Stanislaw Gruszka]

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.

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com> # v3
Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com> # v5
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com>
---
 include/libcamera/internal/v4l2_device.h |  1 +
 src/libcamera/v4l2_device.cpp            | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/libcamera/internal/v4l2_device.h b/include/libcamera/internal/v4l2_device.h
index affe52c2ad3f..a647c96a5eb7 100644
--- a/include/libcamera/internal/v4l2_device.h
+++ b/include/libcamera/internal/v4l2_device.h
@@ -45,6 +45,7 @@ public:
 	const std::string &deviceNode() const { return deviceNode_; }
 	std::string devicePath() const;
 
+	bool supportsFrameStartEvent();
 	int setFrameStartEnabled(bool enable);
 	Signal<uint32_t> frameStart;
 
diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
index 2f65a43a0547..0db92c19ca4a 100644
--- a/src/libcamera/v4l2_device.cpp
+++ b/src/libcamera/v4l2_device.cpp
@@ -449,6 +449,28 @@ std::string V4L2Device::devicePath() const
 	return path;
 }
 
+/**
+ * \brief Check if frame start event is supported
+ *
+ * Due to limitations in the kernel API, this function may disable the frame
+ * start event as a side effect. It should only be called during initialization,
+ * before enabling the frame start event with setFrameStartEnabled().
+ *
+ * \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)
+		return false;
+
+	ioctl(VIDIOC_UNSUBSCRIBE_EVENT, &event);
+	return true;
+}
+
 /**
  * \brief Enable or disable frame start event notification
  * \param[in] enable True to enable frame start events, false to disable them
-- 
2.43.0