[PATCH v4 3/4] RISC-V: Prefer sstc extension if available

[Anup Patel]

On Fri, May 27, 2022 at 9:59 AM Atish Patra <atishp at rivosinc.com> wrote:
>
> RISC-V ISA has sstc extension which allows updating the next clock event
> via a CSR (stimecmp) instead of an SBI call. This should happen dynamically
> if sstc extension is available. Otherwise, it will fallback to SBI call
> to maintain backward compatibility.
>
> Signed-off-by: Atish Patra <atishp at rivosinc.com>
> ---
>  drivers/clocksource/timer-riscv.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index 1767f8bf2013..881d9335c92d 100644
> --- a/drivers/clocksource/timer-riscv.c
> +++ b/drivers/clocksource/timer-riscv.c
> @@ -7,6 +7,9 @@
>   * either be read from the "time" and "timeh" CSRs, and can use the SBI to
>   * setup events, or directly accessed using MMIO registers.
>   */
> +
> +#define pr_fmt(fmt) "timer: " fmt

The "timer: " prefix is too generic. I suggest to use "riscv-timer: "
as a prefix.

> +
>  #include <linux/clocksource.h>
>  #include <linux/clockchips.h>
>  #include <linux/cpu.h>
> @@ -23,11 +26,24 @@
>  #include <asm/sbi.h>
>  #include <asm/timex.h>
>
> +static DEFINE_STATIC_KEY_FALSE(riscv_sstc_available);
> +
>  static int riscv_clock_next_event(unsigned long delta,
>                 struct clock_event_device *ce)
>  {
> +       u64 next_tval = get_cycles64() + delta;
> +
>         csr_set(CSR_IE, IE_TIE);
> -       sbi_set_timer(get_cycles64() + delta);
> +       if (static_branch_likely(&riscv_sstc_available)) {
> +#if defined(CONFIG_32BIT)
> +               csr_write(CSR_STIMECMP, next_tval & 0xFFFFFFFF);
> +               csr_write(CSR_STIMECMPH, next_tval >> 32);
> +#else
> +               csr_write(CSR_STIMECMP, next_tval);
> +#endif
> +       } else
> +               sbi_set_timer(next_tval);
> +
>         return 0;
>  }
>
> @@ -165,6 +181,12 @@ static int __init riscv_timer_init_dt(struct device_node *n)
>         if (error)
>                 pr_err("cpu hp setup state failed for RISCV timer [%d]\n",
>                        error);
> +
> +       if (riscv_isa_extension_available(NULL, SSTC)) {
> +               pr_info("Timer interrupt in S-mode is available via sstc extension\n");
> +               static_branch_enable(&riscv_sstc_available);
> +       }
> +
>         return error;
>  }
>
> --
> 2.25.1
>

Apart from the minor comment above, this looks good to me.

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

Regards,
Anup