Hi,
Here's another attempt at supporting user-space allocations from a specific carved-out reserved memory region.
The initial problem we were discussing was that I'm currently working on a platform which has a memory layout with ECC enabled. However, enabling the ECC has a number of drawbacks on that platform: lower performance, increased memory usage, etc. So for things like framebuffers, the trade-off isn't great and thus there's a memory region with ECC disabled to allocate from for such use cases.
After a suggestion from John, I chose to first start using heap allocations flags to allow for userspace to ask for a particular ECC setup. This is then backed by a new heap type that runs from reserved memory chunks flagged as such, and the existing DT properties to specify the ECC properties.
After further discussion, it was considered that flags were not the right solution, and relying on the names of the heaps would be enough to let userspace know the kind of buffer it deals with.
Thus, even though the uAPI part of it had been dropped in this second version, we still needed a driver to create heaps out of carved-out memory regions. In addition to the original usecase, a similar driver can be found in BSPs from most vendors, so I believe it would be a useful addition to the kernel.
Some extra discussion with Rob Herring [1] came to the conclusion that some specific compatible for this is not great either, and as such an new driver probably isn't called for either.
Some other discussions we had with John [2] also dropped some hints that multiple CMA heaps might be a good idea, and some vendors seem to do that too.
So here's another attempt that doesn't affect the device tree at all and will just create a heap for every CMA reserved memory region.
It also falls nicely into the current plan we have to support cgroups in DRM/KMS and v4l2, which is an additional benefit.
Let me know what you think, Maxime
1: https://lore.kernel.org/all/20250707-cobalt-dingo-of-serenity-dbf92c@houat/ 2: https://lore.kernel.org/all/CANDhNCroe6ZBtN_o=c71kzFFaWK-fF5rCdnr9P5h1sgPOWS...
Let me know what you think, Maxime
Signed-off-by: Maxime Ripard mripard@kernel.org --- Changes in v8: - Rebased on top of 6.18-rc1 - Added TJ R-b - Link to v7: https://lore.kernel.org/r/20250721-dma-buf-ecc-heap-v7-0-031836e1a942@kernel...
Changes in v7: - Invert the logic and register CMA heap from the reserved memory / dma contiguous code, instead of iterating over them from the CMA heap. - Link to v6: https://lore.kernel.org/r/20250709-dma-buf-ecc-heap-v6-0-dac9bf80f35d@kernel...
Changes in v6: - Drop the new driver and allocate a CMA heap for each region now - Dropped the binding - Rebased on 6.16-rc5 - Link to v5: https://lore.kernel.org/r/20250617-dma-buf-ecc-heap-v5-0-0abdc5863a4f@kernel...
Changes in v5: - Rebased on 6.16-rc2 - Switch from property to dedicated binding - Link to v4: https://lore.kernel.org/r/20250520-dma-buf-ecc-heap-v4-1-bd2e1f1bb42c@kernel...
Changes in v4: - Rebased on 6.15-rc7 - Map buffers only when map is actually called, not at allocation time - Deal with restricted-dma-pool and shared-dma-pool - Reword Kconfig options - Properly report dma_map_sgtable failures - Link to v3: https://lore.kernel.org/r/20250407-dma-buf-ecc-heap-v3-0-97cdd36a5f29@kernel...
Changes in v3: - Reworked global variable patch - Link to v2: https://lore.kernel.org/r/20250401-dma-buf-ecc-heap-v2-0-043fd006a1af@kernel...
Changes in v2: - Add vmap/vunmap operations - Drop ECC flags uapi - Rebase on top of 6.14 - Link to v1: https://lore.kernel.org/r/20240515-dma-buf-ecc-heap-v1-0-54cbbd049511@kernel...
--- Maxime Ripard (5): doc: dma-buf: List the heaps by name dma-buf: heaps: cma: Register list of CMA regions at boot dma: contiguous: Register reusable CMA regions at boot dma: contiguous: Reserve default CMA heap dma-buf: heaps: cma: Create CMA heap for each CMA reserved region
Documentation/userspace-api/dma-buf-heaps.rst | 24 ++++++++------ MAINTAINERS | 1 + drivers/dma-buf/heaps/Kconfig | 10 ------ drivers/dma-buf/heaps/cma_heap.c | 47 +++++++++++++++++---------- include/linux/dma-buf/heaps/cma.h | 16 +++++++++ kernel/dma/contiguous.c | 11 +++++++ 6 files changed, 72 insertions(+), 37 deletions(-) --- base-commit: 47633099a672fc7bfe604ef454e4f116e2c954b1 change-id: 20240515-dma-buf-ecc-heap-28a311d2c94e prerequisite-message-id: 20250610131231.1724627-1-jkangas@redhat.com prerequisite-patch-id: bc44be5968feb187f2bc1b8074af7209462b18e7 prerequisite-patch-id: f02a91b723e5ec01fbfedf3c3905218b43d432da prerequisite-patch-id: e944d0a3e22f2cdf4d3b3906e5603af934696deb
Best regards,