linux-kernel - Re: [PATCH 3/3] irqchip: dw-apb-ictl: add PM support

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Message-ID: <20141008195832.1fb16337@xhacker>
Date:	Wed, 8 Oct 2014 19:58:32 +0800
From:	Jisheng Zhang <jszhang@...vell.com>
To:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	"jason@...edaemon.net" <jason@...edaemon.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 3/3] irqchip: dw-apb-ictl: add PM support

Hi Sebastian,

On Wed, 8 Oct 2014 04:50:53 -0700
Jisheng Zhang <jszhang@...vell.com> wrote:

> Hi Sebastian,
> 
> On Wed, 8 Oct 2014 04:44:49 -0700
> Sebastian Hesselbarth <sebastian.hesselbarth@...il.com> wrote:
> 
> > On 10/08/2014 01:31 PM, Jisheng Zhang wrote:
> > > Hi Thomas, Sebastian,
> > >
> > > On Tue, 30 Sep 2014 14:52:54 -0700
> > > Thomas Gleixner <tglx@...utronix.de> wrote:
> > >
> > >> On Tue, 30 Sep 2014, Sebastian Hesselbarth wrote:
> > >>> On 09/23/2014 08:35 AM, Jisheng Zhang wrote:
> > >>>> This patch adds in support for S2R for dw-apb-ictl irqchip driver.
> > >>>>
> > >>>> Signed-off-by: Jisheng Zhang <jszhang@...vell.com>
> > >>>> ---
> > >>>>    drivers/irqchip/irq-dw-apb-ictl.c | 19 +++++++++++++++++++
> > >>>>    1 file changed, 19 insertions(+)
> > >>>>
> > >>>> diff --git a/drivers/irqchip/irq-dw-apb-ictl.c
> > >>>> b/drivers/irqchip/irq-dw-apb-ictl.c
> > >>>> index c136b67..53bb732 100644
> > >>>> --- a/drivers/irqchip/irq-dw-apb-ictl.c
> > >>>> +++ b/drivers/irqchip/irq-dw-apb-ictl.c
> > >>>> @@ -50,6 +50,21 @@ static void dw_apb_ictl_handler(unsigned int irq,
> > >>>> struct irq_desc *desc)
> > >>>>    	chained_irq_exit(chip, desc);
> > >>>>    }
> > >>>>
> > >>>> +#ifdef CONFIG_PM
> > >>>> +static void dw_apb_ictl_resume(struct irq_data *d)
> > >>>> +{
> > >>>> +	struct irq_chip_generic *gc = irq_data_get_irq_chip_data(d);
> > >>>> +	struct irq_chip_type *ct = irq_data_get_chip_type(d);
> > >>>> +
> > >>>> +	irq_gc_lock(gc);
> > >>>> +	writel_relaxed(~0, gc->reg_base + ct->regs.enable);
> > >>>> +	writel_relaxed(*ct->mask_cache, gc->reg_base +
> > >>>> ct->regs.mask);
> > >>>> +	irq_gc_unlock(gc);
> > >>>> +}
> > >>>
> > >>> I agree with the overall change, but may this also be suited for a
> > >>> generic irq_chip helper instead of being a driver specific one?
> > >>>
> > >>> Maybe Thomas or Jason can comment on this.
> > >>
> > >> If we have enough similar resume callbacks, yes.
> > >>
> > >>> Also, now that you are using writel_relaxed, I understand that both
> > >>> writes above can happen in any order? Are there any implication we
> > >>> have to consider, i.e. do we require any of the registers above to
> > >>> be written first?
> > >
> > > The registers sits at device type memory, the writes should happen in
> > > the same order as before.
> > 
> > Jisheng,
> > 
> > it is not about the location of the register but, as far as I
> > understand, when using {readl,writel}_relaxed the compiler is
> > free to reorder the calls. So, if there is a strict order we
> 
> The "volatile" in readl/writel relaxed implementations should prevent the
> compiler to do reorder. Or I misunderstand something?

My understanding is that the relaxed version imply compiler barriers.
I'm not sure I understand the real/writel relaxed implementations correctly. But
one obvious example which shows the relaxed version won't have the compiler
reorder issue is drivers/irqchip/irq-gic.c, all the configurations must be done
before enable the GIC which is done by "writel_relaxed(1, cpu_base + GIC_CPU_CTRL);"
However, we didn't see any explicit compiler barriers.

Thanks,
Jisheng


> 
> Thanks,
> Jisheng
> 
> > want to ensure, we have to use non-relaxed {readl,writel}.
> > 
> > The performance penalty of non-relaxed calls can be ignored anyway
> > as it is done only once after resume.
> > 
> > Sebastian
> 

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