Add SyncPolicy to dma_buf SyncGuard

include/systematic/linux/dma_buf.hpp

@@ -4,7 +4,9 @@
 
 #include <linux/dma-buf.h>
 
+#include <cerrno>
 #include <cstdint>
+#include <system_error>
 #include <thread>
 
 
@@ -24,6 +26,15 @@ namespace systematic :: linux_ :: dma_buf
     }
 
 
+    /// @brief How SyncGuard treats an fd that is not a DMA-BUF.
+    ///
+    enum class SyncPolicy
+    {
+        Strict,     ///< Any ioctl failure throws.
+        BestEffort, ///< A non-DMA-BUF fd (`ENOTTY`) is a no-op; other failures throw.
+    };
+
+
     /// @brief RAII guard bracketing CPU access to a DMA-BUF.
     ///
     /// Calls sync() with `DMA_BUF_SYNC_START | flags` on construction; on
@@ -38,24 +49,43 @@ namespace systematic :: linux_ :: dma_buf
         /// @param flags Access mode (`DMA_BUF_SYNC_READ`, `DMA_BUF_SYNC_WRITE`, or
         ///   `DMA_BUF_SYNC_RW`). Do not pass `DMA_BUF_SYNC_START` / `DMA_BUF_SYNC_END`.
         ///
-        /// @throws std::system_error if `DMA_BUF_IOCTL_SYNC` fails.
+        /// @param policy Whether a non-DMA-BUF fd is an error (`Strict`) or a no-op
+        ///   (`BestEffort`). Use `BestEffort` when the fd may reference plain,
+        ///   already-CPU-coherent memory (e.g. a memfd) that needs no syncing.
+        ///
+        /// @throws std::system_error if `DMA_BUF_IOCTL_SYNC` fails, except `ENOTTY`
+        ///   under `SyncPolicy::BestEffort`.
         ///
-        explicit SyncGuard(int fd, std::uint64_t flags)
+        explicit SyncGuard(int fd, std::uint64_t flags, SyncPolicy policy = SyncPolicy::Strict)
         :   fd_ {fd}
         ,   flags_ {flags}
         {
-            sync(fd_, DMA_BUF_SYNC_START | flags_);
+            ::dma_buf_sync arg { .flags = DMA_BUF_SYNC_START | flags_ };
+
+            if (::ioctl(fd_, DMA_BUF_IOCTL_SYNC, &arg) != 0)
+            {
+                // A non-DMA-BUF fd (e.g. a plain memfd) rejects the ioctl with ENOTTY.
+                // Such memory is already CPU-coherent, so BestEffort treats it as a no-op.
+                if (policy == SyncPolicy::BestEffort && errno == ENOTTY)
+                    active_ = false;
+                else
+                    throw std::system_error {errno, std::system_category(), "DMA_BUF_IOCTL_SYNC failed"};
+            }
         }
 
 
         /// @brief Ends the CPU-access window.
         ///
-        /// ioctl() errors are ignored (return value not checked).
+        /// ioctl() errors are ignored (return value not checked). Does nothing when
+        /// construction found the fd was not a DMA-BUF.
         ///
         ~SyncGuard()
         {
-            ::dma_buf_sync arg { .flags = DMA_BUF_SYNC_END | flags_ };
-            ::ioctl(fd_, DMA_BUF_IOCTL_SYNC, &arg);
+            if (active_)
+            {
+                ::dma_buf_sync arg { .flags = DMA_BUF_SYNC_END | flags_ };
+                ::ioctl(fd_, DMA_BUF_IOCTL_SYNC, &arg);
+            }
         }
 
 
@@ -65,5 +95,6 @@ namespace systematic :: linux_ :: dma_buf
     private:
         int const fd_;
         std::uint64_t const flags_;
+        bool active_ = true;
     };
 }