Allocating dma-buf using heaps

Dma-buf Heaps are a way for userspace to allocate dma-buf objects. They are typically used to allocate buffers from a specific allocation pool, or to share buffers across frameworks.

Heaps

A heap represents a specific allocator. The Linux kernel currently supports the following heaps:

  • The system heap allocates virtually contiguous, cacheable, buffers.

  • The default_cma_region heap allocates physically contiguous, cacheable, buffers. Only present if a CMA region is present. Such a region is usually created either through the kernel commandline through the cma parameter, a memory region Device-Tree node with the linux,cma-default property set, or through the CMA_SIZE_MBYTES or CMA_SIZE_PERCENTAGE Kconfig options. Prior to Linux 6.17, its name wasn’t stable and could be called reserved, linux,cma, or default-pool, depending on the platform.

  • A heap will be created for each reusable region in the device tree with the shared-dma-pool compatible, using the full device tree node name as its name. The buffer semantics are identical to default-cma-region.

Naming Convention

dma-buf heaps name should meet a number of constraints:

  • The name must be stable, and must not change from one version to the other. Userspace identifies heaps by their name, so if the names ever change, we would be likely to introduce regressions.

  • The name must describe the memory region the heap will allocate from, and must uniquely identify it in a given platform. Since userspace applications use the heap name as the discriminant, it must be able to tell which heap it wants to use reliably if there’s multiple heaps.

  • The name must not mention implementation details, such as the allocator. The heap driver will change over time, and implementation details when it was introduced might not be relevant in the future.

  • The name should describe properties of the buffers that would be allocated. Doing so will make heap identification easier for userspace. Such properties are:

    • contiguous for physically contiguous buffers;

    • protected for encrypted buffers not accessible the OS;

  • The name may describe intended usage. Doing so will make heap identification easier for userspace applications and users.

For example, assuming a platform with a reserved memory region located at the RAM address 0x42000000, intended to allocate video framebuffers, physically contiguous, and backed by the CMA kernel allocator, good names would be memory@42000000-contiguous or video@42000000, but cma-video wouldn’t.