linux-kernel - Re: [PATCH] clocksource/drivers/arm_arch_timer: Disable timer before programming CVAL

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Message-ID: <c45cf91e-60fa-c17e-2ec9-bdd7dd64d2dd@collabora.com>
Date:   Mon, 17 Jul 2023 11:22:02 +0200
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     walter.chang@...iatek.com, Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>
Cc:     wsd_upstream@...iatek.com, stanley.chu@...iatek.com,
        Chun-hung.Wu@...iatek.com, Freddy.Hsin@...iatek.com,
        stable@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] clocksource/drivers/arm_arch_timer: Disable timer before
 programming CVAL

Il 17/07/23 11:07, walter.chang@...iatek.com ha scritto:
> From: Walter Chang <walter.chang@...iatek.com>
> 
> Due to the fact that the use of `writeq_relaxed()` to program CVAL is
> not guaranteed to be atomic, it is necessary to disable the timer before
> programming CVAL.
> 
> However, if the MMIO timer is already enabled and has not yet expired,
> there is a possibility of unexpected behavior occurring: when the CPU
> enters the idle state during this period, and if the CPU's local event
> is earlier than the broadcast event, the following process occurs:
> 
> tick_broadcast_enter()
>    tick_broadcast_oneshot_control(TICK_BROADCAST_ENTER)
>      __tick_broadcast_oneshot_control()
>        ___tick_broadcast_oneshot_control()
>          tick_broadcast_set_event()
>            clockevents_program_event()
>              set_next_event_mem()
> 
> During this process, the MMIO timer remains enabled while programming
> CVAL. To prevent such behavior, disable timer explicitly prior to
> programming CVAL.
> 
> Fixes: 8b82c4f883a7 ("clocksource/drivers/arm_arch_timer: Move MMIO timer programming over to CVAL")
> Cc: stable@...r.kernel.org
> Signed-off-by: Walter Chang <walter.chang@...iatek.com>

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>

> ---
>   drivers/clocksource/arm_arch_timer.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index e733a2a1927a..7dd2c615bce2 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -792,6 +792,13 @@ static __always_inline void set_next_event_mem(const int access, unsigned long e
>   	u64 cnt;
>   
>   	ctrl = arch_timer_reg_read(access, ARCH_TIMER_REG_CTRL, clk);
> +
> +	/* Timer must be disabled before programming CVAL */
> +	if (ctrl & ARCH_TIMER_CTRL_ENABLE) {
> +		ctrl &= ~ARCH_TIMER_CTRL_ENABLE;
> +		arch_timer_reg_write(access, ARCH_TIMER_REG_CTRL, ctrl, clk);
> +	}
> +
>   	ctrl |= ARCH_TIMER_CTRL_ENABLE;
>   	ctrl &= ~ARCH_TIMER_CTRL_IT_MASK;
>