[PATCH] arm64: Kconfig: deprecate redundant ARM64_USE_LSE_ATOMICS

[Will Deacon]

[+Marc]

On Tue, Dec 23, 2025 at 12:07:30PM +0100, Lukas Bulwahn wrote:
> From: Lukas Bulwahn <lukas.bulwahn at redhat.com>
> 
> 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