[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/
>> * 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 ;)
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