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Date:   Tue, 27 Dec 2022 17:17:28 -0600
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To:     Prabhakar <prabhakar.csengg@gmail.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Conor Dooley <conor.dooley@microchip.com>,
        Heiko Stuebner <heiko@sntech.de>
Cc:     linux-riscv@lists.infradead.org, Anup Patel <anup@brainfault.org>,
        linux-renesas-soc@vger.kernel.org, linux-kernel@vger.kernel.org,
        Biju Das <biju.das.jz@bp.renesas.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
References: <20221211215843.24024-1-prabhakar.mahadev-lad.rj@bp.renesas.com>
From:   Samuel Holland <samuel@sholland.org>
Subject: Re: [RFC PATCH] clocksource/drivers/riscv: Get rid of
 clocksource_arch_init() callback
In-Reply-To: <20221211215843.24024-1-prabhakar.mahadev-lad.rj@bp.renesas.com>
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On 12/11/22 15:58, Prabhakar wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> 
> Having a clocksource_arch_init() callback always sets vdso_clock_mode to
> VDSO_CLOCKMODE_ARCHTIMER if GENERIC_GETTIMEOFDAY is enabled, this is
> required for the riscv-timer.
> 
> This works for platforms where just riscv-timer clocksource is present.
> On platforms where other clock sources are available we want them to
> register with vdso_clock_mode set to VDSO_CLOCKMODE_NONE.
> 
> On the Renesas RZ/Five SoC OSTM block can be used as clocksource [0], to
> avoid multiple clock sources being registered as VDSO_CLOCKMODE_ARCHTIMER
> move setting of vdso_clock_mode in the riscv-timer driver instead of doing
> this in clocksource_arch_init() callback as done similarly for ARM/64
> architecture.

This is definitely a good change to make, but shouldn't we still prefer
the architectural CSR-based clocksource over an MMIO-based clocksource?
It has double the number of bits, and reading it should have less
overhead. So I think we also want to increase the rating of
riscv_clocksource.

D1 is in the same situation, as timer-sun4i.c registers a clocksource
with a higher rating than riscv_clocksource. Without this patch,
tools/testing/selftests/vDSO/vdso_test_correctness fails. With this
patch, it passes, so:

Tested-by: Samuel Holland <samuel@sholland.org>

> [0] drivers/clocksource/renesas-ostm.c
> 
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> ---
>  arch/riscv/Kconfig                | 1 -
>  arch/riscv/kernel/time.c          | 9 ---------
>  drivers/clocksource/timer-riscv.c | 7 +++++++
>  3 files changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 7ea12de636bd..b269e062c1b1 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -12,7 +12,6 @@ config 32BIT
>  
>  config RISCV
>  	def_bool y
> -	select ARCH_CLOCKSOURCE_INIT
>  	select ARCH_ENABLE_HUGEPAGE_MIGRATION if HUGETLB_PAGE && MIGRATION
>  	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2
>  	select ARCH_HAS_BINFMT_FLAT
> diff --git a/arch/riscv/kernel/time.c b/arch/riscv/kernel/time.c
> index 8217b0f67c6c..42bee305e997 100644
> --- a/arch/riscv/kernel/time.c
> +++ b/arch/riscv/kernel/time.c
> @@ -30,12 +30,3 @@ void __init time_init(void)
>  	of_clk_init(NULL);
>  	timer_probe();
>  }
> -
> -void clocksource_arch_init(struct clocksource *cs)
> -{
> -#ifdef CONFIG_GENERIC_GETTIMEOFDAY
> -	cs->vdso_clock_mode = VDSO_CLOCKMODE_ARCHTIMER;
> -#else
> -	cs->vdso_clock_mode = VDSO_CLOCKMODE_NONE;
> -#endif
> -}
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index 969a552da8d2..7ec9668cd36d 100644
> --- a/drivers/clocksource/timer-riscv.c
> +++ b/drivers/clocksource/timer-riscv.c
> @@ -29,6 +29,12 @@
>  
>  static DEFINE_STATIC_KEY_FALSE(riscv_sstc_available);
>  
> +#ifdef CONFIG_GENERIC_GETTIMEOFDAY
> +static enum vdso_clock_mode vdso_default = VDSO_CLOCKMODE_ARCHTIMER;
> +#else
> +static enum vdso_clock_mode vdso_default = VDSO_CLOCKMODE_NONE;
> +#endif /* CONFIG_GENERIC_GETTIMEOFDAY */

We don't have any workarounds like arm_arch_timer, so we do not need
this indirection through vdso_default. You can set .vdso_clock_mode
directly in the declaration of riscv_clocksource.

Regards,
Samuel

> +
>  static int riscv_clock_next_event(unsigned long delta,
>  		struct clock_event_device *ce)
>  {
> @@ -158,6 +164,7 @@ static int __init riscv_timer_init_dt(struct device_node *n)
>  		return -ENODEV;
>  	}
>  
> +	riscv_clocksource.vdso_clock_mode = vdso_default;
>  	pr_info("%s: Registering clocksource cpuid [%d] hartid [%lu]\n",
>  	       __func__, cpuid, hartid);
>  	error = clocksource_register_hz(&riscv_clocksource, riscv_timebase);