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A dma_addr_t is an address for the device, not a CPU pointer and not necessarily a CPU physical address. The Linux DMA API can translate between device and memory address spaces, so drivers must use the address returned by DMA APIs and follow the mapping’s ownership rules.
Use coherent allocations for shared control structures that CPU and device access over time; use streaming mappings for transfer buffers whose ownership moves between them. The distinction affects allocation lifetime, cache visibility, synchronization, and error handling.
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What a DMA address means
The DMA address is what the device uses to access memory. It may differ from the CPU physical address because an IOMMU or another platform mechanism can translate between address spaces. As the Linux DMA API reference puts it: “A CPU cannot reference a dma_addr_t directly because there may be translation between its physical address space and the DMA address space.” Keep the CPU’s virtual pointer and the device-facing dma_addr_t as separate values; never dereference the DMA address as if it were a CPU pointer.
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Coherent allocations: shared control data
Coherent DMA allocations are suited to memory that the CPU and device access in parallel, such as a descriptor ring, mailbox structure, or firmware data held in main memory. Updates are visible without explicit software cache-flush calls. Drivers commonly allocate and free this memory during device setup and teardown with dma_alloc_coherent() and dma_free_coherent().
Coherent does not mean ordered
Visibility and ordering are different properties. CPU stores may be reordered even when memory is coherent. If the device protocol requires descriptor fields to become visible in a particular order—for example, fields must be written before an ownership or validity flag—use the memory barriers required by that protocol. Coherent allocation alone does not enforce that sequence.
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Many small coherent regions
Coherent memory can be costly, and even a small allocation may consume page-scale resources. For many small, similarly sized coherent objects, consider a dma_pool rather than a collection of independent allocations.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteStreaming mappings: transfer buffers
Streaming mappings are for existing memory used in a device transfer, such as network or storage data. The usual lifecycle is to map the buffer, give the returned DMA address to the device, wait for the transfer to finish, and unmap it. Use dma_map_single() for suitable single-buffer memory, or the page and scatter/gather mapping variants when the memory layout calls for them.
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A mapping can also be kept for reuse, but then the driver must explicitly manage transitions between CPU and device ownership using the appropriate synchronization APIs. The mapping direction must match the actual data flow: DMA_TO_DEVICE is memory to device, DMA_FROM_DEVICE is device to memory, and DMA_BIDIRECTIONAL is for data that can flow both ways or whose direction is not known.
Which mapping model fits?
| Decision point | Coherent allocation | Streaming mapping |
|---|---|---|
| Typical lifetime | Allocated during device setup; freed during teardown | Mapped for a transfer and unmapped afterward, or retained and synchronized for reuse |
| Typical data | Shared control structures, such as descriptor rings and mailbox structures | Transfer buffers, such as network or storage data |
| Visibility and ownership | CPU/device updates are visible without explicit software cache-flush calls; ordering still requires attention | Direction and CPU/device ownership transitions must be handled through mapping and synchronization APIs |
| Common APIs | dma_alloc_coherent(), dma_free_coherent(); dma_pool for many small regions |
dma_map_single(), dma_unmap_single(), page or scatter/gather variants, and sync calls when a mapping is reused |
For example, a driver might keep its descriptor ring in coherent memory for the device’s lifetime, while mapping each packet or storage buffer as a streaming transfer. These are common patterns, not universal rules: the device protocol and actual ownership pattern determine the appropriate choice.
Implementation checks that prevent common DMA bugs
- Set the device’s DMA masks first. Before using DMA, configure the address width the hardware can actually handle, commonly with
dma_set_mask_and_coherent(). Check its return value; if the requested mask is unsupported, do not proceed as though DMA is available. If streaming and coherent DMA have different address limits, configure separate masks as appropriate. See the Linux DMA API HOWTO. - Check every streaming map for failure. A map can fail if the device or an intervening bus cannot address the buffer and the platform cannot provide a suitable IOMMU mapping or bounce buffer. Call
dma_mapping_error()before passing the result to hardware, and handle failure as a normal error path. - Match direction to data flow. Use
DMA_TO_DEVICEwhen the device reads memory,DMA_FROM_DEVICEwhen it writes memory, andDMA_BIDIRECTIONALwhen both directions may occur or are not known. The direction is part of the mapping contract, not merely a label. - Synchronize reused streaming mappings at ownership changes. After the CPU’s final writes and before handing a buffer to the device, perform the required synchronization. Before the CPU reads data the device may have changed, synchronize for CPU access. Bidirectional use requires transitions both ways. If unmapping ends the mapping instead, unmap it with parameters matching the original map.
dma_need_sync()can indicate whether explicit synchronization is needed on the platform. - Use scatter/gather for discontiguous memory.
dma_map_single()does not describe multiple discontiguous segments. Use the scatter/gather API for such buffers. Kernel virtual contiguity alone does not guarantee physical contiguity, and portable drivers cannot assume an IOMMU is present. - Account for cache-line boundaries on non-coherent systems. Cache maintenance operates at cache-line granularity. If the driver cannot safely account for sharing at those boundaries, the API reference recommends page-bounded regions. Follow the constraints of the target platform.
- Consider mapping size on throughput-sensitive paths. The API provides
dma_max_mapping_size()anddma_opt_mapping_size(). For high-rate, short-lived transfers, mapping overhead can be a meaningful part of the request lifetime; where splitting carries no significant penalty, the reference suggests limiting streaming mapping length to the optimal size. These are runtime, platform-specific values, not a universal size threshold.
Check the target kernel’s documentation
DMA API details can change across kernel versions and depend on platform behavior. The links here are to the current Linux kernel documentation; use the API reference and HOWTO corresponding to the kernel you are targeting when implementing or reviewing a driver.
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