Documentation: dma-buf: heaps: Add naming guidelines

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>
Link: https://lore.kernel.org/all/20250728-dma-buf-heap-names-doc-v4-1-f73f71cf0dfd@kernel.org/
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

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

@@ -26,3 +26,37 @@ following heaps:
    ``DMABUF_HEAPS_CMA_LEGACY`` Kconfig option is set, a duplicate node is
    created following legacy naming conventions; the legacy name might be
    ``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.