DMA FENCES OVERVIEW(9)

💅 DMA fences overview(9) 💅

okay so this is a page from the Kernel API Manual, and honestly? it's a whole vibe. bestie, we're talking about fences — not the wooden kind, the sync kind. let's get into it 🫶

✨ NAME ✨

DMA fences overview — the whole entire rundown on how DMA fences work, no gatekeeping 💖

🌸 DESCRIPTION 🌸

So. DMA fences, represented by struct dma_fence, are the kernel's internal synchronization primitive for DMA operations — think GPU rendering, video encoding/decoding, or displaying buffers on a screen. yes literally all of that. it's giving "traffic controller but make it silicon" 👑

A fence gets initialized using dma_fence_init and completed using dma_fence_signal. fences are associated with a context, allocated through dma_fence_context_alloc, and — important — all fences on the same context are fully ordered. no cap. same context = strict order, always 🔥

Since the whole point of fences is to facilitate cross-device and cross-application synchronization, there's multiple ways to use one:

• Individual fences can be exposed as a sync_file, accessed as a file descriptor from userspace, created by calling sync_file_create. this one's called explicit fencing, since userspace passes around explicit synchronization points. very demure, very mindful, very "here is my sync point, hand-delivered" 🎀
• Some subsystems also have their own explicit fencing primitives, like drm_syncobj. compared to sync_file, a drm_syncobj allows the underlying fence to be updated. glow-up behavior fr fr ✨
• Then there's also implicit fencing, where the synchronization points are implicitly passed around as part of shared dma_buf instances. such implicit fences are stored in struct dma_resv through the dma_buf.resv pointer. it's the "it just comes with the bag" of fencing 👜

🦋 SEE ALSO 🦋

the whole friend group, assembly-style 😭 —

• Kernel file ./drivers/dma-buf/dma-fence.c
• fence cross-driver contract(9)
• dma_fence_get_stub(9)
• dma_fence_allocate_private_stub(9)
• dma_fence_context_alloc(9)
• fence signalling annotation(9)
• dma_fence_begin_signalling(9)
• dma_fence_end_signalling(9)
• dma_fence_signal_timestamp_locked(9)
• dma_fence_signal_timestamp(9)
• dma_fence_signal_locked(9)
• dma_fence_check_and_signal_locked(9)
• dma_fence_check_and_signal(9)
• dma_fence_signal(9)
• dma_fence_wait_timeout(9)
• dma_fence_release(9)
• dma_fence_free(9)
• dma_fence_enable_signaling(9)
• dma_fence_add_callback(9)
• dma_fence_get_status(9)
• dma_fence_remove_callback(9)
• dma_fence_default_wait(9)
• dma_fence_wait_any_timeout(9)
• deadline hints(9)
• dma_fence_set_deadline(9)
• dma_fence_describe(9)
• dma_fence_init(9)
• dma_fence_init64(9)
• dma_fence_driver_name(9)
• dma_fence_timeline_name(9)

and that's the tea ☕ this page lives in ./drivers/dma-buf, it's dated October 2026, and honestly? iconic. go read it again if you need to, i'll wait 💜🧚