1.. SPDX-License-Identifier: GPL-2.0 2 3============================== 4Allocating dma-buf using heaps 5============================== 6 7Dma-buf Heaps are a way for userspace to allocate dma-buf objects. They are 8typically used to allocate buffers from a specific allocation pool, or to share 9buffers across frameworks. 10 11Heaps 12===== 13 14A heap represents a specific allocator. The Linux kernel currently supports the 15following heaps: 16 17 - The ``system`` heap allocates virtually contiguous, cacheable, buffers. 18 19 - The ``system_cc_shared`` heap allocates virtually contiguous, cacheable, 20 buffers using shared (decrypted) memory. It is only present on 21 confidential computing (CoCo) VMs where memory encryption is active 22 (e.g., AMD SEV, Intel TDX). The allocated pages have the encryption 23 bit cleared, making them accessible for device DMA without TDISP 24 support. On non-CoCo VM configurations, this heap is not registered. 25 26 - The ``default_cma_region`` heap allocates physically contiguous, 27 cacheable, buffers. Only present if a CMA region is present. Such a 28 region is usually created either through the kernel commandline 29 through the ``cma`` parameter, a memory region Device-Tree node with 30 the ``linux,cma-default`` property set, or through the 31 ``CMA_SIZE_MBYTES`` or ``CMA_SIZE_PERCENTAGE`` Kconfig options. Prior 32 to Linux 6.17, its name wasn't stable and could be called 33 ``reserved``, ``linux,cma``, or ``default-pool``, depending on the 34 platform. 35 36 - A heap will be created for each reusable region in the device tree 37 with the ``shared-dma-pool`` compatible, using the full device tree 38 node name as its name. The buffer semantics are identical to 39 ``default-cma-region``. 40 41Naming Convention 42================= 43 44``dma-buf`` heaps name should meet a number of constraints: 45 46- The name must be stable, and must not change from one version to the other. 47 Userspace identifies heaps by their name, so if the names ever change, we 48 would be likely to introduce regressions. 49 50- The name must describe the memory region the heap will allocate from, and 51 must uniquely identify it in a given platform. Since userspace applications 52 use the heap name as the discriminant, it must be able to tell which heap it 53 wants to use reliably if there's multiple heaps. 54 55- The name must not mention implementation details, such as the allocator. The 56 heap driver will change over time, and implementation details when it was 57 introduced might not be relevant in the future. 58 59- The name should describe properties of the buffers that would be allocated. 60 Doing so will make heap identification easier for userspace. Such properties 61 are: 62 63 - ``contiguous`` for physically contiguous buffers; 64 65 - ``protected`` for encrypted buffers not accessible the OS; 66 67- The name may describe intended usage. Doing so will make heap identification 68 easier for userspace applications and users. 69 70For example, assuming a platform with a reserved memory region located 71at the RAM address 0x42000000, intended to allocate video framebuffers, 72physically contiguous, and backed by the CMA kernel allocator, good 73names would be ``memory@42000000-contiguous`` or ``video@42000000``, but 74``cma-video`` wouldn't. 75