linux-kernel - Re: [PATCH 03/15] arm64: Introduce helpers for page table levels

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Message-ID: <20151008172856.GQ17192@e104818-lin.cambridge.arm.com>
Date:	Thu, 8 Oct 2015 18:28:56 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
Cc:	Christoffer Dall <christoffer.dall@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>, kvm@...r.kernel.org,
	ard.biesheuvel@...aro.org, Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 03/15] arm64: Introduce helpers for page table levels

On Thu, Oct 08, 2015 at 06:22:34PM +0100, Suzuki K. Poulose wrote:
> On 08/10/15 15:45, Christoffer Dall wrote:
> >On Wed, Oct 07, 2015 at 10:26:14AM +0100, Marc Zyngier wrote:
> >>I just had a chat with Catalin, who did shed some light on this.
> >>It all has to do with rounding up. What you would like to have here is:
> >>
> >>#define ARM64_HW_PGTABLE_LEVELS(va_bits) DIV_ROUND_UP(va_bits - PAGE_SHIFT, PAGE_SHIFT - 3)
> >>
> >>where (va_bits - PAGE_SHIFT) is the total number of bits we deal
> >>with during a page table walk, and (PAGE_SHIFT - 3) is the number
> >>of bits we deal with per level.
> >>
> >>The clue is in how DIV_ROUND_UP is written:
> >>
> >>#define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
> >>
> >>which gives you Suzuki's magic formula.
> >>
> >>I'd vote for the DIV_ROUND_UP(), which will make things a lot more readable.
> >>
> >Thanks for the explanation, I vote for DIV_ROUND_UP too.
> 
> Btw, DIV_ROUND_UP is defined in linux/kernel.h, including which in the required
> headers breaks the build. I could add the definition of the same locally.

Or just keep the original magic formula and add the DIV_ROUND_UP one in
a comment.

-- 
Catalin
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