[+Marc]

On Tue, Dec 23, 2025 at 12:07:30PM +0100, Lukas Bulwahn wrote:

From: Lukas Bulwahn <redacted>

Currently, the config options ARM64_USE_LSE_ATOMICS and ARM64_LSE_ATOMICS
are equivalent, i.e., ARM64_LSE_ATOMICS is true if and only if
ARM64_USE_LSE_ATOMICS is true.

Prior to commit 395af861377d ("arm64: Move the LSE gas support detection to
Kconfig")---included in v5.6-rc1---only the config option ARM64_LSE_ATOMICS
was defined, and the check for gas support was done in the Makefile. This
mentioned commit then introduces the config option ARM64_USE_LSE_ATOMICS to
be the promptable option, and changes the semantics of ARM64_LSE_ATOMICS to
check for the gas support.

Note that there is then some minor refactoring in commit 2decad92f473
("arm64: mte: Ensure TIF_MTE_ASYNC_FAULT is set atomically"), putting this
gas support check into its own config option AS_HAS_LSE_ATOMICS, but the
logic remains the same. Since every binutils version defined suitable for
kernel compilation then eventually included the required support, the
config option AS_HAS_LSE_ATOMICS and the dependency was dropped with
commit 2555d4c68720 ("arm64: drop binutils version checks"). This then
makes ARM64_USE_LSE_ATOMICS and ARM64_LSE_ATOMICS equivalent. Hence, one
of the two config options can be dropped now.

Considerations for the decision which config option to drop:

  - ARM64_USE_LSE_ATOMICS is promptable by the user since its introduction
    in 2020. So there might be some Kconfig fragments that define this
    config option and expect that this then implies ARM64_LSE_ATOMICS to be
    set. However, within the kernel tree, there is no existing config file
    referring to that option. So, it is unlikely to be widely used.
  - ARM64_LSE_ATOMICS is used in nine places within the arm64 directory in
    the current kernel tree.
  - ARM64_USE_LSE_ATOMICS is the only config option that contains the infix
    string _USE_ to enable support and use of an arm64 architectural
    feature. However, there is not a very stringent and consistent naming
    convention for Kconfig options throughout the kernel tree anyway.
  - The use of the transitional attribute allows to simplify transitioning
    to a different Kconfig symbol name, but also adds some intermediate
    definition to be removed later eventually.

After thoughtful consideration, keep ARM_LSE_ATOMICS and remove
ARM64_USE_LSE_ATOMICS in a two-step approach, first deprecate
ARM64_USE_LSE_ATOMICS with the transitional attribute here and then plan
to completely remove it in two or three years with a further dedicated
commit then.

Marc was talking about removing ARM64_LSE_ATOMICS entirely the other day
after it bit him with a KVM change. If all supported assemblers understand
the LSE instructions, let's just do that?

Will