[libcamera-devel] [RFC 09/12] libcamera: v4l2_videodevice: Add a buffer cache class

[Niklas Söderlund]

In preparation for the new buffer handling add a class which will deal
with keeping the cache between dmafds and V4L2 video device buffer
indexes. Previously this responsibility have been split between multiple
classes.

This initial implement ensures that no hot association is lost while its
eviction strategy could be improved in the future.

Signed-off-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>
---
 src/libcamera/include/v4l2_videodevice.h | 19 ++++++++++
 src/libcamera/v4l2_videodevice.cpp       | 47 ++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/src/libcamera/include/v4l2_videodevice.h b/src/libcamera/include/v4l2_videodevice.h
index 4b8cf9394eb9516f..5b178339d0ce7e2c 100644
--- a/src/libcamera/include/v4l2_videodevice.h
+++ b/src/libcamera/include/v4l2_videodevice.h
@@ -104,6 +104,25 @@ struct V4L2Capability final : v4l2_capability {
 	}
 };
 
+class V4L2BufferCache
+{
+public:
+	V4L2BufferCache(unsigned int size);
+
+	void populate(unsigned int index, const std::array<int, 3> &fds);
+
+	int pop(const std::array<int, 3> &fds);
+	void push(unsigned int index);
+
+private:
+	struct CacheInfo {
+		bool free;
+		std::array<int, 3> last;
+	};
+
+	std::map<unsigned int, CacheInfo> cache_;
+};
+
 class V4L2DeviceFormat
 {
 public:
diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
index a2a9eab2bcc0d7e8..8bc2e439e4faeb68 100644
--- a/src/libcamera/v4l2_videodevice.cpp
+++ b/src/libcamera/v4l2_videodevice.cpp
@@ -132,6 +132,53 @@ LOG_DECLARE_CATEGORY(V4L2)
  * \return True if the video device provides Streaming I/O IOCTLs
  */
 
+V4L2BufferCache::V4L2BufferCache(unsigned int size)
+{
+	for (unsigned int i = 0; i < size; i++)
+		cache_[i] = { .free = true, .last = { -1, -1, -1 } };
+}
+
+void V4L2BufferCache::populate(unsigned int index, const std::array<int, 3> &fds)
+{
+	ASSERT(index < cache_.size());
+	cache_[index].last = fds;
+}
+
+int V4L2BufferCache::pop(const std::array<int, 3> &fds)
+{
+	int use = -1;
+
+	for (auto &it : cache_) {
+		int index = it.first;
+		CacheInfo &info = it.second;
+
+		if (!info.free)
+			continue;
+
+		if (use < 0)
+			use = index;
+
+		if (info.last == fds) {
+			use = index;
+			break;
+		}
+	}
+
+	if (use < 0)
+		return -ENOENT;
+
+	cache_[use].free = false;
+	cache_[use].last = fds;
+
+	return use;
+}
+
+void V4L2BufferCache::push(unsigned int index)
+{
+	ASSERT(index < cache_.size());
+	cache_[index].free = true;
+}
+
 /**
  * \class V4L2DeviceFormat
  * \brief The V4L2 video device image format and sizes
-- 
2.23.0