Documentation: dma-buf: heaps: Add heap name definitions

Following a recent discussion at last Plumbers, John Stultz, Sumit
Sewal, TJ Mercier and I came to an agreement that we should document
what the dma-buf heaps names are expected to be, and what the buffers
attributes you'll get should be documented.

Let's create that doc to make sure those attributes and names are
guaranteed going forward.

Reviewed-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250306135114.1943738-1-mripard@kernel.org

Author: mripard

Documentation/userspace-api/dma-buf-heaps.rst

@@ -0,0 +1,25 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+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 ``cma`` 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. Depending on the platform, it
+   might be called ``reserved``, ``linux,cma``, or ``default-pool``.

Documentation/userspace-api/index.rst

@@ -44,6 +44,7 @@ Devices and I/O
    :maxdepth: 1
 
    accelerators/ocxl
+   dma-buf-heaps
    dma-buf-alloc-exchange
    gpio/index
    iommufd

MAINTAINERS

@@ -6913,6 +6913,7 @@ L:	dri-devel@lists.freedesktop.org
 L:	linaro-mm-sig@lists.linaro.org (moderated for non-subscribers)
 S:	Maintained
 T:	git https://gitlab.freedesktop.org/drm/misc/kernel.git
+F:	Documentation/userspace-api/dma-buf-heaps.rst
 F:	drivers/dma-buf/dma-heap.c
 F:	drivers/dma-buf/heaps/*
 F:	include/linux/dma-heap.h