Re: [PATCH v2 2/2] clocksource: timer-econet-en751221: fix IRQ ack storm on interrupt handler

[Caleb James DeLisle]

On 13/04/2026 18:04, kpursoty@xxxxxxxxx wrote:
> The interrupt handler acked by writing the current count back to compare:
>
>    iowrite32(ioread32(reg_count(cpu)), reg_compare(cpu));
>
> Since the interrupt fires when count >= compare, and the counter advances
> past the written value before the MMIO write reaches the peripheral, the
> condition re-asserts on the same bus cycle. This causes a tight IRQ storm
> on CPU0, stalling boot at T=62s (measured: boot never progresses past
> clockevents registration).
>
> The EcoNet GPL kernel (arch/mips/econet/time2.c, cputmr_timer_ack()) acks
> by writing the *next* deadline, which puts count < compare and clears the
> pending condition. Write U32_MAX (unreachable before set_next_event()
> programs the real deadline), consistent with cevt_dev_init() which also
> writes U32_MAX to park the compare register at init.
>
> Fixes: 3b4c33ac87d0 ("clocksource/drivers: Add EcoNet Timer HPT driver")
> Signed-off-by: Kervin Pursoty <kpursoty@xxxxxxxxx>

I don't understand the subsystem well enough to know if this is correct, 
but it boots on EN751221.

Tested-by: Caleb James DeLisle <cjd@xxxxxxxx>

> ---
>   drivers/clocksource/timer-econet-en751221.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/clocksource/timer-econet-en751221.c b/drivers/clocksource/timer-econet-en751221.c
> --- a/drivers/clocksource/timer-econet-en751221.c
> +++ b/drivers/clocksource/timer-econet-en751221.c
> @@ -68,6 +68,10 @@ static irqreturn_t cevt_interrupt(int irq, void *dev_id)
>   	if (!cevt_is_pending(cpu))
>   		return IRQ_NONE;
>
> -	iowrite32(ioread32(reg_count(cpu)), reg_compare(cpu));
> +	/*
> +	 * Ack: write a future value so count < compare, clearing the pending
> +	 * condition. set_next_event() will program the real deadline.
> +	 */
> +	iowrite32(U32_MAX, reg_compare(cpu));
>   	dev->event_handler(dev);
>   	return IRQ_HANDLED;
>   }