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 <bagasdo...@gmail.com> Signed-off-by: Maxime Ripard <mrip...@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-v2-1-8ae43174c...@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-ab31f7480...@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..3ee4e7961fe390ba356a2125d53b060546c3e4a6 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-cacheable-contiguous`` or ``video@42000000``, but +``cma-video`` wouldn't. --- base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494 change-id: 20250520-dma-buf-heap-names-doc-31261aa0cfe6 Best regards, -- Maxime Ripard <mrip...@kernel.org>
