[PATCH v3 1/3] riscv: clocksource: Fix stimecmp update hazard on RV32

[Anup Patel]

On Sun, Jan 4, 2026 at 7:30 PM Naohiko Shimizu
<naohiko.shimizu at gmail.com> wrote:
>
> On RV32, updating the 64-bit stimecmp (or vstimecmp) CSR requires two
> separate 32-bit writes. A race condition exists if the timer triggers
> during these two writes.
>
> The RISC-V Privileged Specification (e.g., Section 3.2.1 for mtimecmp)
> recommends a specific 3-step sequence to avoid spurious interrupts
> when updating 64-bit comparison registers on 32-bit systems:
>
> 1. Set the low-order bits (stimecmp) to all ones (ULONG_MAX).
> 2. Set the high-order bits (stimecmph) to the desired value.
> 3. Set the low-order bits (stimecmp) to the desired value.
>
> Current implementation writes the LSB first without ensuring a future
> value, which may lead to a transient state where the 64-bit comparison
> is incorrectly evaluated as "expired" by the hardware. This results in
> spurious timer interrupts.
>
> This patch adopts the spec-recommended 3-step sequence to ensure the
> intermediate 64-bit state is never smaller than the current time.
>
> Fixes: 9f7a8ff6391f ("RISC-V: Prefer sstc extension if available")
> Signed-off-by: Naohiko Shimizu <naohiko.shimizu at gmail.com>

LGTM.

Reviewed-by: Anup Patel <anup at brainfault.org>

Regards,
Anup


> ---
>  drivers/clocksource/timer-riscv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index 4d7cf338824a..cfc4d83c42c0 100644
> --- a/drivers/clocksource/timer-riscv.c
> +++ b/drivers/clocksource/timer-riscv.c
> @@ -50,8 +50,9 @@ static int riscv_clock_next_event(unsigned long delta,
>
>         if (static_branch_likely(&riscv_sstc_available)) {
>  #if defined(CONFIG_32BIT)
> -               csr_write(CSR_STIMECMP, next_tval & 0xFFFFFFFF);
> +               csr_write(CSR_STIMECMP, ULONG_MAX);
>                 csr_write(CSR_STIMECMPH, next_tval >> 32);
> +               csr_write(CSR_STIMECMP, next_tval & 0xFFFFFFFF);
>  #else
>                 csr_write(CSR_STIMECMP, next_tval);
>  #endif
> --
> 2.39.5
>