Public bug reported:
[Impact]
The amd-embedded kernel uses the Ubuntu default CONFIG_CMA_ALIGNMENT=8.
With 4 KiB pages, CMA-backed contiguous DMA buffers on amd64 are then
aligned to at most 1 MiB. Drivers and workloads on AMD embedded
platforms that need larger naturally aligned contiguous buffers cannot
rely on alignment above 1 MiB.
[Fix]
Set CONFIG_CMA_ALIGNMENT=12 on amd64 for the amd-embedded and
amd-embedded-rt flavours. This raises the maximum alignment of
contiguous DMA buffers to 16 MiB. arm64 keeps the default value of 8.
The change only affects the debian.amd-embedded config annotations.
[Test Plan]
1. Build the amd-embedded and amd-embedded-rt kernels for amd64 and
the amd-embedded kernel for arm64.
2. Check that /boot/config-* contains CONFIG_CMA_ALIGNMENT=12 for both
amd64 kernels and CONFIG_CMA_ALIGNMENT=8 for the arm64 kernel.
[Where problems could occur]
A higher maximum alignment can make large CMA allocations more
sensitive to memory fragmentation. Large allocations may then fail or
take longer when the CMA area is small or fragmented. Allocations of
1 MiB or less are not affected. On amd64 the default CMA area is empty,
so the change only matters when a CMA area is configured.
** Affects: linux (Ubuntu)
Importance: Undecided
Assignee: Krystian Kaniewski (kkaniewski)
Status: Confirmed
** Affects: linux (Ubuntu Resolute)
Importance: High
Assignee: Krystian Kaniewski (kkaniewski)
Status: Confirmed
** Also affects: linux (Ubuntu Resolute)
Importance: Undecided
Status: New
** Changed in: linux (Ubuntu Resolute)
Assignee: (unassigned) => Krystian Kaniewski (kkaniewski)
** Changed in: linux (Ubuntu)
Assignee: (unassigned) => Krystian Kaniewski (kkaniewski)
** Changed in: linux (Ubuntu Resolute)
Status: New => Confirmed
** Changed in: linux (Ubuntu)
Status: New => Confirmed
** Changed in: linux (Ubuntu Resolute)
Importance: Undecided => High
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https://bugs.launchpad.net/bugs/2172552
Title:
amd-embedded: Raise CONFIG_CMA_ALIGNMENT to 12 on amd64
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