Re: [PATCH v7 3/4] RISC-V: Prefer sstc extension if available

[Guo Ren]

Looks good to me. The patch is concise, and static_branch is properly used.

Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>

On Sat, Jul 23, 2022 at 12:51 AM Atish Patra <atishp@xxxxxxxxxxxx> 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.
>
> Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>
> Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx>
> ---
>  drivers/clocksource/timer-riscv.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index 593d5a957b69..05f6cf067289 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) "riscv-timer: " fmt
> +
>  #include <linux/clocksource.h>
>  #include <linux/clockchips.h>
>  #include <linux/cpu.h>
> @@ -20,14 +23,28 @@
>  #include <linux/of_irq.h>
>  #include <clocksource/timer-riscv.h>
>  #include <asm/smp.h>
> +#include <asm/hwcap.h>
>  #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 +182,12 @@ 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
>


-- 
Best Regards
 Guo Ren