[PATCH v4] Documentation: dma-buf: heaps: Add naming guidelines

[Maxime Ripard <mripard@kernel.org>]

We've discussed a number of times of how some heap names are bad, but
not really what makes a good heap name.

Let's document what we expect the heap names to look like.

Reviewed-by: Andrew Davis <afd@ti.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
---
Changes in v4:
- Dropped *all* the cacheable mentions
- Link to v3: https://lore.kernel.org/r/20250717-dma-buf-heap-names-doc-v3-1-d2dbb4b95ef6@kernel.org

Changes in v3:
- Grammar, spelling fixes
- Remove the cacheable / uncacheable name suggestion
- Link to v2: https://lore.kernel.org/r/20250616-dma-buf-heap-names-doc-v2-1-8ae43174cdbf@kernel.org

Changes in v2:
- Added justifications for each requirement / suggestions
- Added a mention and example of buffer attributes
- Link to v1: https://lore.kernel.org/r/20250520-dma-buf-heap-names-doc-v1-1-ab31f74809ee@kernel.org
---
 Documentation/userspace-api/dma-buf-heaps.rst | 35 +++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/Documentation/userspace-api/dma-buf-heaps.rst b/Documentation/userspace-api/dma-buf-heaps.rst
index 535f49047ce6450796bf4380c989e109355efc05..1ced2720f929432661182f1a3a88aa1ff80bd6af 100644
--- a/Documentation/userspace-api/dma-buf-heaps.rst
+++ b/Documentation/userspace-api/dma-buf-heaps.rst
@@ -21,5 +21,40 @@ following heaps:
    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``.
+
+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.

---
base-commit: 038d61fd642278bab63ee8ef722c50d10ab01e8f
change-id: 20250520-dma-buf-heap-names-doc-31261aa0cfe6

Best regards,
-- 
Maxime Ripard <mripard@kernel.org>