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2034134de6
Since we're going to introduce multiple instances of the CMA heap driver, there's no single CMA heap anymore. Let's use the heap name instead to differentiate between all the heaps available in the system. While we're at it, let's also rework the backward compatibility part to make it easier to amend later on. Reviewed-by: T.J. Mercier <tjmercier@google.com> Signed-off-by: Maxime Ripard <mripard@kernel.org> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org> [sumits: rebased to latest Doc] Link: https://lore.kernel.org/r/20251013-dma-buf-ecc-heap-v8-1-04ce150ea3d9@kernel.org
65 lines
2.7 KiB
ReStructuredText
65 lines
2.7 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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==============================
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Allocating dma-buf using heaps
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==============================
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Dma-buf Heaps are a way for userspace to allocate dma-buf objects. They are
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typically used to allocate buffers from a specific allocation pool, or to share
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buffers across frameworks.
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Heaps
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=====
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A heap represents a specific allocator. The Linux kernel currently supports the
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following heaps:
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- The ``system`` heap allocates virtually contiguous, cacheable, buffers.
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- The ``default_cma_region`` heap allocates physically contiguous,
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cacheable, buffers. Only present if a CMA region is present. Such a
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region is usually created either through the kernel commandline
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through the ``cma`` parameter, a memory region Device-Tree node with
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the ``linux,cma-default`` property set, or through the
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``CMA_SIZE_MBYTES`` or ``CMA_SIZE_PERCENTAGE`` Kconfig options. Prior
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to Linux 6.17, its name wasn't stable and could be called
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``reserved``, ``linux,cma``, or ``default-pool``, depending on the
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platform. From Linux 6.17 onwards, the creation of these heaps is
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controlled through the ``DMABUF_HEAPS_CMA_LEGACY`` Kconfig option for
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backwards compatibility.
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Naming Convention
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=================
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``dma-buf`` heaps name should meet a number of constraints:
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- The name must be stable, and must not change from one version to the other.
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Userspace identifies heaps by their name, so if the names ever change, we
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would be likely to introduce regressions.
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- The name must describe the memory region the heap will allocate from, and
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must uniquely identify it in a given platform. Since userspace applications
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use the heap name as the discriminant, it must be able to tell which heap it
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wants to use reliably if there's multiple heaps.
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- The name must not mention implementation details, such as the allocator. The
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heap driver will change over time, and implementation details when it was
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introduced might not be relevant in the future.
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- The name should describe properties of the buffers that would be allocated.
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Doing so will make heap identification easier for userspace. Such properties
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are:
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- ``contiguous`` for physically contiguous buffers;
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- ``protected`` for encrypted buffers not accessible the OS;
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- The name may describe intended usage. Doing so will make heap identification
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easier for userspace applications and users.
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For example, assuming a platform with a reserved memory region located
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at the RAM address 0x42000000, intended to allocate video framebuffers,
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physically contiguous, and backed by the CMA kernel allocator, good
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names would be ``memory@42000000-contiguous`` or ``video@42000000``, but
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``cma-video`` wouldn't.
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