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

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Date:   Mon, 17 Jul 2023 17:07:34 +0800
From:   <walter.chang@...iatek.com>
To:     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>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
CC:     <wsd_upstream@...iatek.com>, <stanley.chu@...iatek.com>,
        <Chun-hung.Wu@...iatek.com>, <Freddy.Hsin@...iatek.com>,
        <walter.chang@...iatek.com>, <stable@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>
Subject: [PATCH] clocksource/drivers/arm_arch_timer: Disable timer before programming CVAL

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;
 
-- 
2.18.0