linux-kernel - Re: [PATCH v3 1/6] iommu/core: split mapping to page sizes as supported by the hardware

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Message-ID: <CAK=WgbYG2tpMJ2SikZhm=R9j5kQ6Hm8CGg=q1bko3idzF53Q3Q@mail.gmail.com>
Date:	Tue, 27 Sep 2011 16:28:37 +0300
From:	Ohad Ben-Cohen <ohad@...ery.com>
To:	"Roedel, Joerg" <Joerg.Roedel@....com>
Cc:	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	Hiroshi DOYU <Hiroshi.DOYU@...ia.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	David Woodhouse <dwmw2@...radead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	David Brown <davidb@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stepan Moskovchenko <stepanm@...eaurora.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 1/6] iommu/core: split mapping to page sizes as
 supported by the hardware

On Tue, Sep 27, 2011 at 4:12 PM, Roedel, Joerg <Joerg.Roedel@....com> wrote:
> You pass a pointer to an unsigned long for the page-size bitmap. This
> allows to use an array of unsigned long. But a single unsigned long is
> sufficient

This is fine; I can change that if you like it better (though without
doing the change below this is probably moot).

> and you can use functions like ffs() and fls() together with
> shifting. These functions often translate to a single intruction in the
> binary. The find_next_bit function has much more overhead because it
> needs to handle the array-of-ulong case.

So you're suggesting to re-implement find_next_bit() using ffs()/fls()
and shifting ?

What's the point ?

Sure, if we'll have a proven performance issue while using
find_next_bit() we can think of doing this, but at this stage, this
sounds to me like a premature optimization which isn't too elegant.

At this point I strongly prefer readable, maintainable and easier to
debug code over optimization which isn't proven to be necessary.

Thanks,
Ohad.
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