2 files changed, 20 insertions, 10 deletions

diff --git a/src/libcamera/include/v4l2_videodevice.h b/src/libcamera/include/v4l2_videodevice.h
index 359b3664..04802012 100644
--- a/src/libcamera/include/v4l2_videodevice.h
+++ b/src/libcamera/include/v4l2_videodevice.h
@@ -7,11 +7,12 @@
 #ifndef __LIBCAMERA_V4L2_VIDEODEVICE_H__
 #define __LIBCAMERA_V4L2_VIDEODEVICE_H__
 
+#include <atomic>
+#include <memory>
 #include <string>
 #include <vector>
 
 #include <linux/videodev2.h>
-#include <memory>
 
 #include <libcamera/buffer.h>
 #include <libcamera/geometry.h>
@@ -120,11 +121,12 @@ private:
 	{
 	public:
 		Entry();
-		Entry(bool free, const FrameBuffer &buffer);
+		Entry(bool free, uint64_t lastUsed, const FrameBuffer &buffer);
 
 		bool operator==(const FrameBuffer &buffer) const;
 
 		bool free;
+		uint64_t lastUsed;
 
 	private:
 		struct Plane {
@@ -140,6 +142,7 @@ private:
 		std::vector<Plane> planes_;
 	};
 
+	std::atomic_uint64_t lastUsedCounter_;
 	std::vector<Entry> cache_;
 	/* \todo Expose the miss counter through an instrumentation API. */
 	unsigned int missCounter_;
diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
index c495de85..ca0d147f 100644
--- a/src/libcamera/v4l2_videodevice.cpp
+++ b/src/libcamera/v4l2_videodevice.cpp
@@ -162,7 +162,7 @@ LOG_DECLARE_CATEGORY(V4L2)
  * buffer import, with buffers added to the cache as they are queued.
  */
 V4L2BufferCache::V4L2BufferCache(unsigned int numEntries)
-	: missCounter_(0)
+	: lastUsedCounter_(1), missCounter_(0)
 {
 	cache_.resize(numEntries);
 }
@@ -176,10 +176,12 @@ V4L2BufferCache::V4L2BufferCache(unsigned int numEntries)
  * allocated.
  */
 V4L2BufferCache::V4L2BufferCache(const std::vector<std::unique_ptr<FrameBuffer>> &buffers)
-	: missCounter_(0)
+	: lastUsedCounter_(1), missCounter_(0)
 {
 	for (const std::unique_ptr<FrameBuffer> &buffer : buffers)
-		cache_.emplace_back(true, buffer->planes());
+		cache_.emplace_back(true,
+				    lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
+				    buffer->planes());
 }
 
 V4L2BufferCache::~V4L2BufferCache()
@@ -205,6 +207,7 @@ int V4L2BufferCache::get(const FrameBuffer &buffer)
 {
 	bool hit = false;
 	int use = -1;
+	uint64_t oldest = UINT64_MAX;
 
 	for (unsigned int index = 0; index < cache_.size(); index++) {
 		const Entry &entry = cache_[index];
@@ -219,8 +222,10 @@ int V4L2BufferCache::get(const FrameBuffer &buffer)
 			break;
 		}
 
-		if (use < 0)
+		if (entry.lastUsed < oldest) {
 			use = index;
+			oldest = entry.lastUsed;
+		}
 	}
 
 	if (!hit)
@@ -229,7 +234,9 @@ int V4L2BufferCache::get(const FrameBuffer &buffer)
 	if (use < 0)
 		return -ENOENT;
 
-	cache_[use] = Entry(false, buffer);
+	cache_[use] = Entry(false,
+			    lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
+			    buffer);
 
 	return use;
 }
@@ -245,12 +252,12 @@ void V4L2BufferCache::put(unsigned int index)
 }
 
 V4L2BufferCache::Entry::Entry()
-	: free(true)
+	: free(true), lastUsed(0)
 {
 }
 
-V4L2BufferCache::Entry::Entry(bool free, const FrameBuffer &buffer)
-	: free(free)
+V4L2BufferCache::Entry::Entry(bool free, uint64_t lastUsed, const FrameBuffer &buffer)
+	: free(free), lastUsed(lastUsed)
 {
 	for (const FrameBuffer::Plane &plane : buffer.planes())
 		planes_.emplace_back(plane);