dma-contiguous: add kconfig option to setup numa cma area if not configured explicitly

There was a report on a multi-numa-nodes arm64 server that when IOMMU
is disabled, the dma_alloc_coherent() function always returns memory
from node 0 even for devices attaching to other nodes, while they can
get local dma memory when IOMMU is on with the same API.

The reason is, when IOMMU is disabled, the dma_alloc_coherent() will
go the direct way and call dma_alloc_contiguous(). The system doesn't
have any explicit cma setting (like per-numa cma), and only has a
default 64MB cma reserved area (on node 0), where kernel will try
first to allocate memory from.

Robin Murphy suggested to setup pernuma cma or disable cma, which did
solve the issue. While there is still concern that for customers
which don't have much kernel knowledge, they could still suffer from
this silently as some architectures enable cma area by default (not
an issue for X86 though, which set CONFIG_CMA_SIZE_MBYTES to 0 by
default) for most Linux distributions.

One thought is to follow the current cma reserving policy for platform
with 'CONFIG_DMA_NUMA_CMA=y', that if the numa cma (either the 'numa cma'
or 'cma pernuma' method) is not explicitly configured, and the platform
really has multiple NUMA nodes, set it up according to size of default
'dma_contiguous_default_area'. This way, the default behavior of
platform with one NUMA node is kept unchanged (say embedded/small
devices don't need to allocate extra memory), while the general dma
locality is improved.

Add a new bool kernel config CONFIG_CMA_SIZE_PERNUMA to control whether
to enable it. Even when the config is enabled, user can still disable
it by kernel-cmdline setting like "numa_cma=0:0" or "cma_pernuma=0".

Reported-by: Changrong Chen <chenchangrong.ccr@alibaba-inc.com>
Suggested-by: Ying Huang <ying.huang@linux.alibaba.com>
Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Feng Tang <feng.tang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20260512085509.83002-1-feng.tang@linux.alibaba.com
Link: https://lore.kernel.org/all/20260520222742.GA1607511@ax162/
[mszyprow: squashed changes from both links, added __initdata attribute
           to the numa_cma_configured variable]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

0 files changed, 0 insertions, 0 deletions