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

Suzuki K. Poulose Suzuki.Poulose at arm.com
Thu Oct 15 06:48:47 PDT 2015

```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 at linaro.org>
>

Thanks. I lost two interview questions though ;)

Suzuki

```