Hello Maxime,

On Mon, 28 Jul 2025 at 13:51, Maxime Ripard mripard@kernel.org wrote:

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.

Thank you for the patch. In principle, I'm ok to take this patch, with the obvious understanding that if there are future heap name requirements that can't be satisfied with these rules, we will discuss and adapt the rules accordingly.

I hope this sounds reasonable to all.

If I don't hear any objections, I'll merge this by this weekend.

Reviewed-by: Andrew Davis afd@ti.com Reviewed-by: Bagas Sanjaya bagasdotme@gmail.com Signed-off-by: Maxime Ripard mripard@kernel.org

Best. Sumit.

---

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@...

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@...

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@...

---

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