linux-kernel - Re: x86: Clean up computation of HPET .mult variables

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Message-ID: <20080506125920.GA26295@beyonder.ift.unesp.br>
Date:	Tue, 6 May 2008 09:59:20 -0300
From:	"Carlos R. Mafra" <crmafra2@...il.com>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	venkatesh.pallipadi@...el.com
Subject: Re: x86: Clean up computation of HPET .mult variables

On Mon  5.May'08 at 20:23:38 -0700, Daniel Walker wrote:
> 
> On Mon, 2008-05-05 at 23:13 -0300, Carlos R. Mafra wrote:
> > So the savings in my patch is due to using the period directly, and
> > not the frequency. That's what my idea was, so if you object then
> > my attempt was a failure and should be forgotten :-)
> > 
> > Or maybe I should create a clocksource_period2mult to replace 
> > clocksource_hz2mult and save the extra operation in more places too?
> 
> The one concern I have is the rounding that is done in the
> clocksource_hz2mult(). The div_sc doesn't include it .. 

So that would be a point in favour of using div_sc(), right?

> You could add a
> clocksource_period2mult(), that would help out any one later that has a
> period instead of hz ..

Hmm, clocksource_period2mult() would be just a rename of div_sc(), see
for example how clocksource_hpet.mult is computed with my patch:

clocksource_hpet.mult = div_sc(hpet_period, FSEC_PER_NSEC, HPET_SHIFT);

However, hpet_clockevent.mult would also require the exchange of
the first two arguments, due to the different definition of 'mult' in
clockchips.h and clocksource.h

So I would like to ask if this different definition of mult 
variables in clockevent versus clocksource is intentional or not.

And do you agree that your first suggestion of using clocksource_hz2mult
makes the code a bit bigger due to the extra computation of the frequency?

My patch saves 49 bytes, and I thought that being careful in the code
comments would make this change a safe thing (because everyone will
understand how the computation is done and that there is a difference
in the definitions).

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