Messages in this thread | | | Subject | Re: [PATCH 03/15] arm64: Introduce helpers for page table levels | From | "Suzuki K. Poulose" <> | Date | Fri, 9 Oct 2015 10:22:23 +0100 |
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On 08/10/15 18:28, Catalin Marinas wrote: > 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. >
OK, will keep proper documentation with the cryptic formula ;)
Suzuki
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