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

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Date:   Fri, 18 Aug 2023 09:29:37 +0000
From:   Walter Chang (張維哲) 
        <Walter.Chang@...iatek.com>
To:     "maz@...nel.org" <maz@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        wsd_upstream <wsd_upstream@...iatek.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Chun-Hung Wu (巫駿宏) 
        <Chun-hung.Wu@...iatek.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Stanley Chu (朱原陞) 
        <stanley.chu@...iatek.com>,
        Freddy Hsin (辛恒豐) 
        <Freddy.Hsin@...iatek.com>
Subject: Re: [PATCH] clocksource/drivers/arm_arch_timer: Disable timer before
 programming CVAL

On Mon, 2023-07-17 at 17:07 +0800, walter.chang@...iatek.com wrote:
> 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>
> ---
>  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;
>  

Gentle ping for this patch.

Thanks,
Walter Chang