[libcamera-devel] [RFC PATCH] libcamera: Request: Add setHalfLife

[Paul Elder]

>From the perspective of a Buffer, a Request can suddenly become nullptr
in the middle of a stream. As Requests are supposed to be atomic, and
they are the nucleus of the libcamera camera streaming API, it would be
convenient if we could set the rate of decay of Requests.

Add a half life member variable to Request, and a setHalfLife() member
function to set this. This will affect whether the Request is reusable
or not, so Request::reuse() can now fail.

Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>

---
TODO: change the applications to reuse the request if reuse() fails
---
 include/libcamera/request.h |  8 +++++++-
 src/libcamera/request.cpp   | 36 ++++++++++++++++++++++++++++++++++--
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/include/libcamera/request.h b/include/libcamera/request.h
index 4cf5ff3f..45c707f3 100644
--- a/include/libcamera/request.h
+++ b/include/libcamera/request.h
@@ -7,6 +7,7 @@
 #ifndef __LIBCAMERA_REQUEST_H__
 #define __LIBCAMERA_REQUEST_H__
 
+#include <chrono>
 #include <map>
 #include <memory>
 #include <stdint.h>
@@ -43,7 +44,7 @@ public:
 	Request(Camera *camera, uint64_t cookie = 0);
 	~Request();
 
-	void reuse(ReuseFlag flags = Default);
+	bool reuse(ReuseFlag flags = Default);
 
 	ControlList &controls() { return *controls_; }
 	ControlList &metadata() { return *metadata_; }
@@ -57,6 +58,8 @@ public:
 
 	bool hasPendingBuffers() const { return !pending_.empty(); }
 
+	void setHalfLife(std::chrono::duration<double> hl);
+
 	std::string toString() const;
 
 private:
@@ -79,6 +82,9 @@ private:
 	const uint64_t cookie_;
 	Status status_;
 	bool cancelled_;
+
+	std::chrono::steady_clock::time_point birthTime_;
+	std::chrono::duration<double> halfLife_;
 };
 
 } /* namespace libcamera */
diff --git a/src/libcamera/request.cpp b/src/libcamera/request.cpp
index ce2dd7b1..0543abaf 100644
--- a/src/libcamera/request.cpp
+++ b/src/libcamera/request.cpp
@@ -7,7 +7,9 @@
 
 #include <libcamera/request.h>
 
+#include <chrono>
 #include <map>
+#include <random>
 #include <sstream>
 
 #include <libcamera/buffer.h>
@@ -74,7 +76,9 @@ LOG_DEFINE_CATEGORY(Request)
  */
 Request::Request(Camera *camera, uint64_t cookie)
 	: camera_(camera), sequence_(0), cookie_(cookie),
-	  status_(RequestPending), cancelled_(false)
+	  status_(RequestPending), cancelled_(false),
+	  birthTime_(std::chrono::steady_clock::now()),
+	  halfLife_(std::chrono::seconds(10))
 {
 	/**
 	 * \todo Should the Camera expose a validator instance, to avoid
@@ -111,11 +115,28 @@ Request::~Request()
  * prior to queueing the request to the camera, in lieu of constructing a new
  * request. The application can reuse the buffers that were previously added
  * to the request via addBuffer() by setting \a flags to ReuseBuffers.
+ *
+ * The request for reuse may fail, as Requests can decay based on the half
+ * life.
+ *
+ * \return True on success, false otherwise
  */
-void Request::reuse(ReuseFlag flags)
+bool Request::reuse(ReuseFlag flags)
 {
 	LIBCAMERA_TRACEPOINT(request_reuse, this);
 
+	std::chrono::steady_clock::time_point end =
+		std::chrono::steady_clock::now();
+	std::chrono::duration<double> diff = end - birthTime_;
+
+	std::random_device rd{};
+	std::mt19937 gen{ rd() };
+	double hlns =
+		std::chrono::duration_cast<std::chrono::nanoseconds>(halfLife_).count();
+	std::normal_distribution<> dist{ hlns, hlns / 2 };
+	if (dist(gen) > hlns)
+		return false;
+
 	pending_.clear();
 	if (flags & ReuseBuffers) {
 		for (auto pair : bufferMap_) {
@@ -133,6 +154,8 @@ void Request::reuse(ReuseFlag flags)
 
 	controls_->clear();
 	metadata_->clear();
+
+	return true;
 }
 
 /**
@@ -320,6 +343,15 @@ bool Request::completeBuffer(FrameBuffer *buffer)
 	return !hasPendingBuffers();
 }
 
+/**
+ * \brief Set the half life of the request
+ * \param[in] hl Half-life of the request
+ */
+void Request::setHalfLife(std::chrono::duration<double> hl)
+{
+	halfLife_ = hl;
+}
+
 /**
  * \brief Generate a string representation of the Request internals
  *
-- 
2.27.0