linux-kernel - Re: [PATCH v2] clocksource: exynos-mct: Register the timer for stable udelay

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Message-ID: <CADGdYn6ZCvnfod3smE_mdJhK-BdkHMtno2GCH8jzX=BRWDCRPQ@mail.gmail.com>
Date:	Fri, 20 Jun 2014 16:49:47 +0530
From:	amit daniel kachhap <amit.daniel@...sung.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Tomasz Figa <tomasz.figa@...il.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	David Riley <davidriley@...omium.org>,
	Jonathan Austin <Jonathan.Austin@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Doug Anderson <dianders@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] clocksource: exynos-mct: Register the timer for stable udelay

On Fri, Jun 20, 2014 at 1:45 PM, Will Deacon <will.deacon@....com> wrote:
> On Thu, Jun 19, 2014 at 05:40:49PM +0100, Tomasz Figa wrote:
>> On 19.06.2014 18:31, Doug Anderson wrote:
>> >>> My personal vote would be to submit a patch to change "cycles_t" to
>> >>> always be 32-bits.  Given that 32-bits was fine for udelay() for ARM
>> >>> that seems sane and simple.  If someone later comes up with a super
>> >>> compelling reason why we need 64-bit timers for udelay (really??) then
>> >>> they can later add all the complexity needed.
>> >>
>> >> Yes, this could work. I'm not sure what else cycles_t is used for, though.
>> >
>> > True, it is a bit questionable to change this since it's a type that's
>> > not obviously just for udelay().  Perhaps a better option would be to
>> > make a new typedef for the result of read_current_timer().  ...or just
>> > change it to return a u32?
>> >
>>
>> Sounds good to me, but let's hear other opinions. I'm adding Will and
>> Jonathan as they wrote the ARM delay timer code.
>
> I think cycles_t is only used for small interval calculations at the moment,
> but I remember Ted mentioning something about using it (or something
> similar) as a source of early entropy, in which case the more bits the
> better.
>
Will,
Thanks for the clarification that cycles_t is used for short
intervals. So it is safe to return lower 32 bit
counter for read_current_timer.

Tomasz, Doug,
As of now let me send a minimal implementation of this read delay
timer to fix the broken udelay for exynos platforms so that it goes to
upstream in rc releases. I will also prepare a fix for all
raw_readl/writel in mct to relaxed version to make it consistent.

> That said, I can't find any code in the tree to that effect.
>
> Will
>
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