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

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Date:   Tue, 14 Jun 2022 10:49:00 -0700
From:   Atish Patra <atishp@...shpatra.org>
To:     Anup Patel <anup@...infault.org>
Cc:     Atish Patra <atishp@...osinc.com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Jisheng Zhang <jszhang@...nel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v4 3/4] RISC-V: Prefer sstc extension if available

On Thu, Jun 9, 2022 at 9:41 PM Anup Patel <anup@...infault.org> wrote:
>
> On Fri, May 27, 2022 at 9:59 AM Atish Patra <atishp@...osinc.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@...osinc.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.
>

Sured. Fixed in the next version.

> > +
> >  #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@...infault.org>
>
> Regards,
> Anup



-- 
Regards,
Atish