Messages in this thread | | | Subject | Re: [PATCH 03/15] arm64: Introduce helpers for page table levels | From | "Suzuki K. Poulose" <> | Date | Thu, 8 Oct 2015 18:22:34 +0100 |
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On 08/10/15 15:45, Christoffer Dall wrote: > On Wed, Oct 07, 2015 at 10:26:14AM +0100, Marc Zyngier wrote: >> On 07/10/15 09:26, Christoffer Dall wrote: >>> Hi Suzuki, >>>
>> 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.
> > You can stash this away for a cryptic interview question ;)
;)
Suzuki
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