[libcamera-devel] [RFC 09/11] libcamera: media_device: Add functions to lock device for other processes

[Niklas Söderlund]

Add lock() and unlock() which backed by lockf() allows an instance of
libcamera to claim exclusive access to a media device. These functions
is the base to allow multiple users of libcamera to coexist in the
system without stepping on each others toes.

Signed-off-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>
---
 src/libcamera/include/media_device.h |  4 ++++
 src/libcamera/media_device.cpp       | 29 +++++++++++++++++++++++++++-
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/src/libcamera/include/media_device.h b/src/libcamera/include/media_device.h
index 5d28a7bb8214ef4a..e2956549fa201ea3 100644
--- a/src/libcamera/include/media_device.h
+++ b/src/libcamera/include/media_device.h
@@ -64,6 +64,10 @@ private:
 	bool acquire();
 	void release();
 
+	bool lockOwner_;
+	bool lock();
+	void unlock();
+
 	std::map<unsigned int, MediaObject *> objects_;
 	MediaObject *object(unsigned int id);
 	bool addObject(MediaObject *object);
diff --git a/src/libcamera/media_device.cpp b/src/libcamera/media_device.cpp
index 46931f52688f6c82..5cea4e63715125c8 100644
--- a/src/libcamera/media_device.cpp
+++ b/src/libcamera/media_device.cpp
@@ -62,7 +62,8 @@ LOG_DEFINE_CATEGORY(MediaDevice)
  * populate() before the media graph can be queried.
  */
 MediaDevice::MediaDevice(const std::string &deviceNode)
-	: deviceNode_(deviceNode), fd_(-1), valid_(false), acquired_(false)
+	: deviceNode_(deviceNode), fd_(-1), valid_(false), acquired_(false),
+	  lockOwner_(false)
 {
 }
 
@@ -421,6 +422,32 @@ void MediaDevice::release()
 	acquired_ = false;
 }
 
+bool MediaDevice::lock()
+{
+	if (fd_ == -1)
+		return false;
+
+	if (lockOwner_)
+		return true;
+
+	if (lockf(fd_, F_TLOCK, 0))
+		return false;
+
+	lockOwner_ = true;
+
+	return true;
+}
+
+void MediaDevice::unlock()
+{
+	if (fd_ == -1)
+		return;
+
+	lockOwner_ = false;
+
+	lockf(fd_, F_ULOCK, 0);
+}
+
 /**
  * \var MediaDevice::objects_
  * \brief Global map of media objects (entities, pads, links) keyed by their
-- 
2.21.0