pipeline_handler: Add acquireDevice() function to mirror releaseDevice()

libcamera always keeps all /dev/video# and /dev/v4l2_subdev# nodes for a
pipeline open after enumerating the camera.

This is a problem for the uvcvideo pipeline handler. Keeping /dev/video#
open stops the UVC USB device from being able to enter runtime-suspend
causing significant unnecessary power-usage.

Add a stub acquireDevice() function to the PipelineHandler class which
pipeline handlers can override.

The uvcvideo pipeline handler will use this to delay opening /dev/video#
until the device is acquired. This is a special case because the kernel
uvcvideo driver powers on the USB device as soon as /dev/video# is
opened. This behavior should *not* be copied by other pipeline handlers.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Harvey Yang <chenghaoyang@chromium.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/internal/pipeline_handler.h

@@ -45,7 +45,7 @@ public:
 	MediaDevice *acquireMediaDevice(DeviceEnumerator *enumerator,
 					const DeviceMatch &dm);
 
-	bool acquire();
+	bool acquire(Camera *camera);
 	void release(Camera *camera);
 
 	virtual std::unique_ptr<CameraConfiguration> generateConfiguration(Camera *camera,
@@ -79,6 +79,7 @@ protected:
 	virtual int queueRequestDevice(Camera *camera, Request *request) = 0;
 	virtual void stopDevice(Camera *camera) = 0;
 
+	virtual bool acquireDevice(Camera *camera);
 	virtual void releaseDevice(Camera *camera);
 
 	CameraManager *manager_;

src/libcamera/camera.cpp

@@ -995,7 +995,7 @@ int Camera::acquire()
 	if (ret < 0)
 		return ret == -EACCES ? -EBUSY : ret;
 
-	if (!d->pipe_->acquire()) {
+	if (!d->pipe_->acquire(this)) {
 		LOG(Camera, Info)
 			<< "Pipeline handler in use by another process";
 		return -EBUSY;

src/libcamera/pipeline_handler.cpp

@@ -163,21 +163,25 @@ MediaDevice *PipelineHandler::acquireMediaDevice(DeviceEnumerator *enumerator,
  * has already acquired it
  * \sa release()
  */
-bool PipelineHandler::acquire()
+bool PipelineHandler::acquire(Camera *camera)
 {
 	MutexLocker locker(lock_);
 
-	if (useCount_) {
-		++useCount_;
-		return true;
-	}
-
+	if (useCount_ == 0) {
 		for (std::shared_ptr<MediaDevice> &media : mediaDevices_) {
 			if (!media->lock()) {
 				unlockMediaDevices();
 				return false;
 			}
 		}
+	}
+
+	if (!acquireDevice(camera)) {
+		if (useCount_ == 0)
+			unlockMediaDevices();
+
+		return false;
+	}
 
 	++useCount_;
 	return true;
@@ -213,12 +217,40 @@ void PipelineHandler::release(Camera *camera)
 	--useCount_;
 }
 
+/**
+ * \brief Acquire resources associated with this camera
+ * \param[in] camera The camera for which to acquire resources
+ *
+ * Pipeline handlers may override this in order to get resources such as opening
+ * devices and allocating buffers when a camera is acquired.
+ *
+ * This is used by the uvcvideo pipeline handler to delay opening /dev/video#
+ * until the camera is acquired to avoid excess power consumption. The delayed
+ * opening of /dev/video# is a special case because the kernel uvcvideo driver
+ * powers on the USB device as soon as /dev/video# is opened. This behavior
+ * should *not* be copied by other pipeline handlers.
+ *
+ * \return True on success, false on failure
+ * \sa releaseDevice()
+ */
+bool PipelineHandler::acquireDevice([[maybe_unused]] Camera *camera)
+{
+	return true;
+}
+
 /**
  * \brief Release resources associated with this camera
  * \param[in] camera The camera for which to release resources
  *
  * Pipeline handlers may override this in order to perform cleanup operations
  * when a camera is released, such as freeing memory.
+ *
+ * This is called once for every camera that is released. If there are resources
+ * shared by multiple cameras then the pipeline handler must take care to not
+ * release them until releaseDevice() has been called for all previously
+ * acquired cameras.
+ *
+ * \sa acquireDevice()
  */
 void PipelineHandler::releaseDevice([[maybe_unused]] Camera *camera)
 {