Re: [PATCH 1/2] ARM: local timers: Unmask interrupt before new TVALis set

[Rohit Vaswani]

On 8/11/2012 3:17 AM, Marc Zyngier wrote:
> On Fri, 10 Aug 2012 14:57:34 -0700, Rohit Vaswani
> <rvaswani@xxxxxxxxxxxxxx>
> wrote:
> > Level triggered interrupt is deasserted when a new TVAL is written
> > only when the interrupt is unmasked. Make sure that the interrupt
> > is unmasked in CTL register before TVAL is written.
> > If this order is not followed, there are chances that on some
> > hardware you would not receive any timer interrupts.
> >
> > Signed-off-by: Rohit Vaswani <rvaswani@xxxxxxxxxxxxxx>
> > ---
> >   arch/arm/kernel/arch_timer.c |    2 +-
> >   1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/arm/kernel/arch_timer.c b/arch/arm/kernel/arch_timer.c
> > index dd58035..1d0d9df 100644
> > --- a/arch/arm/kernel/arch_timer.c
> > +++ b/arch/arm/kernel/arch_timer.c
> > @@ -126,8 +126,8 @@ static int arch_timer_set_next_event(unsigned long
> evt,
> >   	ctrl |= ARCH_TIMER_CTRL_ENABLE;
> >   	ctrl &= ~ARCH_TIMER_CTRL_IT_MASK;
> >   
> > -	arch_timer_reg_write(ARCH_TIMER_REG_TVAL, evt);
> >   	arch_timer_reg_write(ARCH_TIMER_REG_CTRL, ctrl);
> > +	arch_timer_reg_write(ARCH_TIMER_REG_TVAL, evt);
> But by doing so, you're opening a window where TVAL can be negative (from
> a previous timer trigger) and the interrupt unmasked, which would lead to
> an immediate trigger, before TVAL is updated with the new value.
>
> Does your hardware deassert the interrupt even when the enable bit is not
> set? If so, would the following sequence work?
>
>          ctrl &= ~(ARCH_TIMER_CTRL_ENABLE | ARCH_TIMER_CTRL_IT_MASK);
>          arch_timer_reg_write(ARCH_TIMER_REG_CTRL, ctrl);
>
> 	arch_timer_reg_write(ARCH_TIMER_REG_TVAL, evt);
>
>          ctrl |= ARCH_TIMER_CTRL_ENABLE;
>          arch_timer_reg_write(ARCH_TIMER_REG_CTRL, ctrl);
>
> Thanks,
>
>          M.
Thanks Marc, this works. I will re-send the patch with these changes.

Thanks,
Rohit Vaswani

--
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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