libcamera: v4l2_videodevice: lastUsedCounter_ need not be atomic

The `V4L2BufferCache` type is not thread-safe. Its `lastUsedCounter_`
member is not used in contexts where its atomicity would matter.
So it does not need to be have an atomic type.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/v4l2_videodevice.h

@@ -8,7 +8,6 @@
 #pragma once
 
 #include <array>
-#include <atomic>
 #include <memory>
 #include <optional>
 #include <ostream>
@@ -158,7 +157,7 @@ private:
 		std::vector<Plane> planes_;
 	};
 
-	std::atomic<uint64_t> lastUsedCounter_;
+	uint64_t lastUsedCounter_;
 	std::vector<Entry> cache_;
 	/* \todo Expose the miss counter through an instrumentation API. */
 	unsigned int missCounter_;

src/libcamera/v4l2_videodevice.cpp

@@ -190,7 +190,7 @@ V4L2BufferCache::V4L2BufferCache(const std::vector<std::unique_ptr<FrameBuffer>>
 {
 	for (const std::unique_ptr<FrameBuffer> &buffer : buffers)
 		cache_.emplace_back(true,
-				    lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
+				    lastUsedCounter_++,
 				    *buffer.get());
 }
 
@@ -258,7 +258,7 @@ int V4L2BufferCache::get(const FrameBuffer &buffer)
 		return -ENOENT;
 
 	cache_[use] = Entry(false,
-			    lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
+			    lastUsedCounter_++,
 			    buffer);
 
 	return use;