[PATCH v3 2/2] riscv: Add Zawrs support for spinlocks

[Conor Dooley]

Hey Heiko,

On Sun, May 21, 2023 at 01:47:15PM +0200, Heiko Stuebner wrote:
> From: Christoph Müllner <christoph.muellner at vrull.eu>
> 
> The current RISC-V code uses the generic ticket lock implementation,
> that calls the macros smp_cond_load_relaxed() and smp_cond_load_acquire().
> Currently, RISC-V uses the generic implementation of these macros.
> This patch introduces a RISC-V specific implementation, of these
> macros, that peels off the first loop iteration and modifies the waiting
> loop such, that it is possible to use the WRS.STO instruction of the Zawrs
> ISA extension to stall the CPU.
> 
> The resulting implementation of smp_cond_load_*() will only work for
> 32-bit or 64-bit types for RV64 and 32-bit types for RV32.
> This is caused by the restrictions of the LR instruction (RISC-V only
> has LR.W and LR.D). Compiler assertions guard this new restriction.
> 
> This patch uses the existing RISC-V ISA extension framework
> to detect the presence of Zawrs at run-time.
> If available a NOP instruction will be replaced by WRS.NTO or WRS.STO.
> 
> The whole mechanism is gated by Kconfig setting, which defaults to Y.
> 
> The Zawrs specification can be found here:
> https://github.com/riscv/riscv-zawrs/blob/main/zawrs.adoc
> 
> Signed-off-by: Christoph Müllner <christoph.muellner at vrull.eu>
> [rebase, update to review comments]
> Signed-off-by: Heiko Stuebner <heiko.stuebner at vrull.eu>

I do intend actually reviewing these two, but busy this week with dt
stuff! In the interim, got some build complaints..

gcc-13 & clang-16 allmodconfig:
kernel/rcu/rcuscale.c:819:3: warning: logical not is only applied to the left hand side of this comparison [-Wlogical-not-parentheses]

k210_nommu_defconfig:
include/linux/atomic/atomic-arch-fallback.h:249:23: error: implicit declaration of function 'smp_load_acquire' [-Werror=implicit-function-declaration]
include/linux/atomic/atomic-arch-fallback.h:265:17: error: implicit declaration of function 'smp_store_release' [-Werror=implicit-function-declaration]

Thanks,
Conor.
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