Message-ID: <1351256204.16863.56.camel@twins>
Subject: Re: [PATCH 01/16] math128: Introduce various 128bit primitives
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Juri Lelli <juri.lelli@gmail.com>, mingo@redhat.com,
        rostedt@goodmis.org, oleg@redhat.com, fweisbec@gmail.com,
        darren@dvhart.com, johan.eker@ericsson.com, p.faure@akatech.ch,
        linux-kernel@vger.kernel.org, claudio@evidence.eu.com,
        michael@amarulasolutions.com, fchecconi@gmail.com,
        tommaso.cucinotta@sssup.it, nicola.manica@disi.unitn.it,
        luca.abeni@unitn.it, dhaval.giani@gmail.com, hgu1972@gmail.com,
        paulmck@linux.vnet.ibm.com, raistlin@linux.it, insop.song@ericsson.com,
        liming.wang@windriver.com, jkacur@redhat.com,
        harald.gustafsson@ericsson.com, vincent.guittot@linaro.org,
        Andrew Morton <akpm@linux-foundation.org>
Date: Fri, 26 Oct 2012 14:56:44 +0200
In-Reply-To: <1351248244.16863.13.camel@twins>
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On Fri, 2012-10-26 at 12:44 +0200, Peter Zijlstra wrote:
> > We can still have the user space interface handing in the information
> > in nsec resolution, but it's reasonable to scale it down to something
> > useful. Just shift the incoming information right by 10, so you're in
> > the 1us resolution for all the internal math and all your limitation
> > problems are gone. A shift by ten for converting back and forth to
> > nsecs is not a real performance issue.
> 
> I'm fine with that.. all I wanted was to not have the undefined overflow
> we initially had. 

Note that we still need the constraint checking with this, although with
both values shifted right 10 bits the range is now much bigger and
shouldn't be a practical limit anymore.
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