Message-Id: <20250616-dma-buf-heap-names-doc-v2-1-8ae43174cdbf@kernel.org>
Date: Mon, 16 Jun 2025 17:21:46 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Sumit Semwal <sumit.semwal@...aro.org>, 
 Benjamin Gaignard <benjamin.gaignard@...labora.com>, 
 Brian Starkey <Brian.Starkey@....com>, John Stultz <jstultz@...gle.com>, 
 "T.J. Mercier" <tjmercier@...gle.com>, Jonathan Corbet <corbet@....net>
Cc: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
 linaro-mm-sig@...ts.linaro.org, linux-doc@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Jared Kangas <jkangas@...hat.com>, 
 Mattijs Korpershoek <mkorpershoek@...nel.org>, 
 Bagas Sanjaya <bagasdotme@...il.com>, Maxime Ripard <mripard@...nel.org>
Subject: [PATCH v2] 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: Bagas Sanjaya <bagasdotme@...il.com>
Signed-off-by: Maxime Ripard <mripard@...nel.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 | 38 +++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/Documentation/userspace-api/dma-buf-heaps.rst b/Documentation/userspace-api/dma-buf-heaps.rst
index 535f49047ce6450796bf4380c989e109355efc05..835ad1c3a65bc07b6f41d387d85c57162909e859 100644
--- a/Documentation/userspace-api/dma-buf-heaps.rst
+++ b/Documentation/userspace-api/dma-buf-heaps.rst
@@ -21,5 +21,43 @@ 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:
+
+- That 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
+  changes, we would be likely to introduce regressions.
+
+- That 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.
+
+- That 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:
+
+  - ``cacheable`` / ``uncacheable`` for buffers with CPU caches enabled
+    or disabled;
+
+  - ``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@...00000-cacheable-contiguous`` or
+``video@...00000``, but ``cma-video`` wouldn't.

---
base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494
change-id: 20250520-dma-buf-heap-names-doc-31261aa0cfe6

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

Powered by blists - more mailing lists