drivers: video: Add get_volatile_ctrl driver's API

Add get_volatile_ctrl() driver's API to retrieve the current value of a
control marked as volatile, e.g. gain, exposure. This function triggers
a hardware register reading instead of returning a cached value to ensure
that users always get a fresh value which is constantly updated by the HW.

Note that the driver is responsible for marking a control as volatile by
setting VIDEO_CTRL_FLAG_VOLATILE when registering a control because not
all hardwares work the same way for the same control.

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>

Author: ngphibang

drivers/video/video_ctrls.c

@@ -151,6 +151,17 @@ int video_get_ctrl(const struct device *dev, struct video_control *control)
 		return -EACCES;
 	}
 
+	if (ctrl->flags & VIDEO_CTRL_FLAG_VOLATILE) {
+		if (DEVICE_API_GET(video, ctrl->vdev->dev)->get_volatile_ctrl == NULL) {
+			return -ENOSYS;
+		}
+
+		/* Call driver's get_volatile_ctrl */
+		return DEVICE_API_GET(video, ctrl->vdev->dev)
+			->get_volatile_ctrl(ctrl->vdev->dev, control);
+	}
+
+	/* Read control value in cache memory */
 	if (ctrl->type == VIDEO_CTRL_TYPE_INTEGER64) {
 		control->val64 = ctrl->val64;
 	} else {

include/zephyr/drivers/video.h

@@ -318,12 +318,12 @@ typedef int (*video_api_flush_t)(const struct device *dev, enum video_endpoint_i
 typedef int (*video_api_set_stream_t)(const struct device *dev, bool enable);
 
 /**
- * @typedef video_api_set_ctrl_t
- * @brief Set a video control value.
+ * @typedef video_api_ctrl_t
+ * @brief Set/Get a video control value.
  *
- * See video_set_ctrl() for argument descriptions.
+ * See video_set_ctrl() or video_get_ctrl() for argument descriptions.
  */
-typedef int (*video_api_set_ctrl_t)(const struct device *dev, struct video_control *ctrl);
+typedef int (*video_api_ctrl_t)(const struct device *dev, struct video_control *ctrl);
 
 /**
  * @typedef video_api_get_caps_t
@@ -353,7 +353,8 @@ __subsystem struct video_driver_api {
 	video_api_enqueue_t enqueue;
 	video_api_dequeue_t dequeue;
 	video_api_flush_t flush;
-	video_api_set_ctrl_t set_ctrl;
+	video_api_ctrl_t set_ctrl;
+	video_api_ctrl_t get_volatile_ctrl;
 	video_api_set_signal_t set_signal;
 	video_api_set_frmival_t set_frmival;
 	video_api_get_frmival_t get_frmival;