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

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Date:	Thu, 15 Oct 2015 10:35:38 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	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, christoffer.dall@...aro.org
Subject: Re: [PATCHv3 03/11] arm64: Introduce helpers for page table levels

On 14/10/15 18:07, Mark Rutland wrote:
> On Wed, Oct 14, 2015 at 12:20:26PM +0100, Suzuki K. Poulose wrote:

>> + * Number of page-table levels required to address 'va_bits' wide
>> + * address, without section mapping. We resolve the top (va_bits - PAGE_SHIFT)
>> + * bits with (PAGE_SHIFT - 3) bits at each page table level. Hence:
>> + *
>> + *  levels = DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - 3))
>> + *
>> + * We cannot include linux/kernel.h which defines DIV_ROUND_UP here
>> + * due to build issues. So we use the following magic formula.
>> + */
>> +#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - 4) / (PAGE_SHIFT - 3))
>
> I think I failed the interview question [1]. :(
>
> I read the comment to mean this was a brand-new piece of magic, as
> opposed to a constant-folded copy of DIV_ROUND_UP. So it seems there's
> still some scope for confusion, even if that only includes me.
>

Wouldn't it be better to modify the comment to say, we open coded the DIV_ROUND_UP ?
We could potentially end up in a conflict if somebody else does __DIV_ROUND_UP.
I have seen similar issues with the CPU feature series, where if I include one
particular header file in another, kernel build breaks without giving you a clue,
what caused the error. Usually due to the multiple definitions (e.g NSEC_PER_SEC)
and other conflicts. Given that this header file gets included with asm/page.h and
hence would be used included for people outside arch/arm64, I would prefer, not to
head there, instead update the comment, something like this :


/*
  * Number of page-table levels required to address 'va_bits' wide
  * address, without section mapping. We resolve the top (va_bits - PAGE_SHIFT)
  * bits with (PAGE_SHIFT - 3) bits at each page table level. Hence:
  *
  *  levels = DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - 3))
  *
  * where DIV_ROUND_UP (n, d) = > ((n) + (d) - 1) / (d)
  *
  * We cannot include linux/kernel.h which defines DIV_ROUND_UP here
  * due to build issues. So we open code the DIV_ROUND_UP and hence
  * we get :
  *  ((va_bits - PAGE_SHIFT) + (PAGE_SHIFT - 3) -1) / (PAGE_SHIFT - 3)
  *
  * which gets simplified as :
  *  (((va_bits) - 4) / (PAGE_SHIFT - 3))
  *
  */

Let me know if you are happy with that ?

Thanks
Suzuki

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