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

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Message-ID: <20140620081509.GA25104@arm.com>
Date:	Fri, 20 Jun 2014 09:15:09 +0100
From:	Will Deacon <will.deacon@....com>
To:	Tomasz Figa <tomasz.figa@...il.com>
Cc:	Doug Anderson <dianders@...omium.org>,
	Amit Daniel Kachhap <amit.daniel@...sung.com>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Riley <davidriley@...omium.org>,
	Jonathan Austin <Jonathan.Austin@....com>
Subject: Re: [PATCH v2] clocksource: exynos-mct: Register the timer for
 stable udelay

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.

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

Will
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