DmaBufAllocator: Add Dma Buffer synchronization function & helper class

To synchronize CPU access with mmap and hardware access on DMA buffers,
using `DMA_BUF_IOCTL_SYNC` is required. This patch adds a function and
a helper class to allow users to sync buffers more easily.

Signed-off-by: Han-Lin Chen <hanlinchen@chromium.org>
Co-developed-by: Harvey Yang <chenghaoyang@chromium.org>
Signed-off-by: Harvey Yang <chenghaoyang@chromium.org>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/dma_buf_allocator.h

@@ -12,6 +12,7 @@
 #include <vector>
 
 #include <libcamera/base/flags.h>
+#include <libcamera/base/shared_fd.h>
 #include <libcamera/base/unique_fd.h>
 
 namespace libcamera {
@@ -48,6 +49,26 @@ private:
 	DmaBufAllocatorFlag type_;
 };
 
+class DmaSyncer final
+{
+public:
+	enum class SyncType {
+		Read = 0,
+		Write,
+		ReadWrite,
+	};
+
+	explicit DmaSyncer(SharedFD fd, SyncType type = SyncType::ReadWrite);
+
+	~DmaSyncer();
+
+private:
+	void sync(uint64_t step);
+
+	SharedFD fd_;
+	uint64_t flags_ = 0;
+};
+
 LIBCAMERA_FLAGS_ENABLE_OPERATORS(DmaBufAllocator::DmaBufAllocatorFlag)
 
 } /* namespace libcamera */