Re: [PATCHv3 03/11] arm64: Introduce helpers for page table levels

[Suzuki K. Poulose]

On 15/10/15 14:30, Christoffer Dall wrote:
> On Thu, Oct 15, 2015 at 02:14:22PM +0100, Suzuki K. Poulose wrote:
> > On 15/10/15 13:44, Mark Rutland wrote:
> > > On Thu, Oct 15, 2015 at 01:37:35PM +0200, Christoffer Dall wrote:
> > > > On Wed, Oct 14, 2015 at 12:20:26PM +0100, Suzuki K. Poulose wrote:


> >   * The maximum number of levels supported by the architecture is 4. Hence at starting
> >   * at level n, we hanve (4 - n) levels of translation. So, the total number of bits
>
> nit: s/hanve/have/

Fixed.

> >   * mapped by an entry at level n is :
> >   *
> >   *  ((4 - n) - 1) * (PAGE_SHIFT - 3) + PAGE_SHIFT
> >   *
> >   * Rearranging it a bit we get :
> >   *   (4 - n) * (PAGE_SHIFT - 3) + 3
> >   */
> >
> > Or we could use the formula without rearranging.
> Either way, even I get it now.
>
> Thanks for the explanation!!

:). I was involved too much in these calculations that, the formula looked
obvious to me, when I wrote it. But yes, I did realise that it is indeed
complicated, once I really started looking at explaining why I wrote it so.
Thanks for being patient :) and complaining peacefully !

Btw, I have a revised (hopefully better) version here :

/*
 * Size mapped by an entry at level n ( 0 <= n <= 3)
 * We map (PAGE_SHIFT - 3) at all translation levels and PAGE_SHIFT bits
 * in the final page. The maximum number of translation levels supported by
 * the architecture is 4. Hence, starting at at level n, we have further
 * ((4 - n) - 1) levels of translation excluding the offset within the page.
 * So, the total number of bits mapped by an entry at level n is :
 *
 *  ((4 - n) - 1) * (PAGE_SHIFT - 3) + PAGE_SHIFT
 *
 * Rearranging it a bit we get :
 *   (4 - n) * (PAGE_SHIFT - 3) + 3
 */


> Assuming some version of this goes in:
>
> Acked-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>

Thanks. I lost two interview questions though ;)

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/