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

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Message-ID: <561FAEBF.2090508@arm.com>
Date:	Thu, 15 Oct 2015 14:48:47 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	Mark Rutland <mark.rutland@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	catalin.marinas@....com, will.deacon@....com,
	steve.capper@...aro.org, marc.zyngier@....com,
	ard.biesheuvel@...aro.org
Subject: Re: [PATCHv3 03/11] arm64: Introduce helpers for page table levels

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@...aro.org>
>

Thanks. I lost two interview questions though ;)

Suzuki

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