libcamera: media_device: Add DeviceInfo features

Add the features of the DeviceInfo class needed to replace it with
MediaDevice.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/include/media_device.h

@@ -24,6 +24,10 @@ public:
 	MediaDevice(const std::string &devnode);
 	~MediaDevice();
 
+	bool acquire();
+	void release() { acquired_ = false; }
+	bool busy() const { return acquired_; }
+
 	int open();
 	void close();
 
@@ -41,6 +45,7 @@ private:
 	std::string devnode_;
 	int fd_;
 	bool valid_;
+	bool acquired_;
 
 	std::map<unsigned int, MediaObject *> objects_;
 	MediaObject *object(unsigned int id);

src/libcamera/media_device.cpp

@@ -53,6 +53,10 @@ namespace libcamera {
  * This will not invalidate the media graph and all cached media objects remain
  * valid and can be accessed normally. The device can then be later reopened if
  * needed to perform other operations that interact with the device node.
+ *
+ * Media device can be claimed for exclusive use with acquire(), released with
+ * release() and tested with busy(). This mechanism is aimed at pipeline
+ * managers to claim media devices they support during enumeration.
  */
 
 /**
@@ -63,7 +67,7 @@ namespace libcamera {
  * populated with open() and populate() before the media graph can be queried.
  */
 MediaDevice::MediaDevice(const std::string &devnode)
-	: devnode_(devnode), fd_(-1), valid_(false)
+	: devnode_(devnode), fd_(-1), valid_(false), acquired_(false)
 {
 }
 
@@ -74,6 +78,50 @@ MediaDevice::~MediaDevice()
 	clear();
 }
 
+/**
+ * \brief Claim a device for exclusive use
+ *
+ * The device claiming mechanism offers simple media device access arbitration
+ * between multiple users. When the media device is created, it is available to
+ * all users. Users can query the media graph to determine whether they can
+ * support the device and, if they do, claim the device for exclusive use. Other
+ * users are then expected to skip over media devices in use as reported by the
+ * busy() function.
+ *
+ * Once claimed the device shall be released by its user when not needed anymore
+ * by calling the release() function.
+ *
+ * Exclusive access is only guaranteed if all users of the media device abide by
+ * the device claiming mechanism, as it isn't enforced by the media device
+ * itself.
+ *
+ * \return true if the device was successfully claimed, or false if it was
+ * already in use
+ * \sa release(), busy()
+ */
+bool MediaDevice::acquire()
+{
+	if (acquired_)
+		return false;
+
+	acquired_ = true;
+	return true;
+}
+
+/**
+ * \fn MediaDevice::release()
+ * \brief Release a device previously claimed for exclusive use
+ * \sa acquire(), busy()
+ */
+
+/**
+ * \fn MediaDevice::busy()
+ * \brief Check if a device is in use
+ * \return true if the device has been claimed for exclusive use, or false if it
+ * is available
+ * \sa acquire(), release()
+ */
+
 /**
  * \brief Open a media device and retrieve device information
  *