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@gmail.com Signed-off-by: Maxime Ripard mripard@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@... --- 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@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,
On 6/16/25 8:21 AM, Maxime Ripard 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.
Reviewed-by: Bagas Sanjaya bagasdotme@gmail.com Signed-off-by: Maxime Ripard mripard@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@...
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:
For these points below, I would s/That name/The name/ (3 places).
+- 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.
change,
+- 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.
if there are
+- 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
^^^ Not a complete sentence. Change '.' to ',': 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,
linaro-mm-sig@lists.linaro.org
-
Maxime Ripard -
Randy Dunlap