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

[Suzuki K. Poulose]

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/