[PATCH 03/15] arm64: Introduce helpers for page table levels
Suzuki K. Poulose
Suzuki.Poulose at arm.com
Thu Oct 8 10:22:34 PDT 2015
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 ;)
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