STRUCT DMA_BUF_SYNC(9)

hiii bestie 💅 welcome to struct dma_buf_sync(9) from the Kernel API Manual — we're literally about to make CPU access and DMA buffers get along ✨

🌸 NAME 🌸

struct dma_buf_sync — Synchronize with CPU access. no cap, that's the whole vibe check 🫶

✨ SYNOPSIS ✨

struct dma_buf_sync {
    __u64 flags;
 };

🧩 Members 🧩

• flags — the set of access flags 👑
• DMA_BUF_SYNC_START — Indicates the start of a map access session. 🎬
• DMA_BUF_SYNC_END — Indicates the end of a map access session. 🎀
• DMA_BUF_SYNC_READ — Indicates that the mapped DMA buffer will be read by the client via the CPU map. 📖
• DMA_BUF_SYNC_WRITE — Indicates that the mapped DMA buffer will be written by the client via the CPU map. ✍️
• DMA_BUF_SYNC_RW — An alias for DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE. 💅

💫 Description 💫

Okay so picture this: when a DMA buffer is accessed from the CPU via mmap, it is not always possible to guarantee coherency between the CPU-visible map and underlying memory. 😭 To manage coherency, DMA_BUF_IOCTL_SYNC must be used to bracket any CPU access to give the kernel the chance to shuffle memory around if needed. ✨

Prior to accessing the map, the client must call DMA_BUF_IOCTL_SYNC with DMA_BUF_SYNC_START and the appropriate read/write flags. Once the access is complete, the client should call DMA_BUF_IOCTL_SYNC with DMA_BUF_SYNC_END and the same read/write flags. 🫶

The synchronization provided via DMA_BUF_IOCTL_SYNC only provides cache coherency. It does not prevent other processes or devices from accessing the memory at the same time. ⚠️ If synchronization with a GPU or other device driver is required, it is the client's responsibility to wait for buffer to be ready for reading or writing before calling this ioctl with DMA_BUF_SYNC_START. Likewise, the client must ensure that follow-up work is not submitted to GPU or other device driver until after this ioctl has been called with DMA_BUF_SYNC_END? (yes bestie, that question mark is part of the sentence energy 😭)

If the driver or API with which the client is interacting uses implicit synchronization, waiting for prior work to complete can be done via poll on the DMA buffer file descriptor. 🦋 If the driver or API requires explicit synchronization, the client may have to wait on a sync_file or other synchronization primitive outside the scope of the DMA buffer API. 👑

👀 SEE ALSO 👀

• Kernel file ./include/uapi/linux/dma-buf.h
• struct dma_buf_export_sync_file(9)
• struct dma_buf_import_sync_file(9)

okay that's the page bestie 🎀 source/version: ./include/uapi/linux; date: October 2026; page: struct dma_buf_sync(9). mwah 💋