From: Suzuki.Poulose@arm•com (Suzuki K. Poulose)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCHv3 03/11] arm64: Introduce helpers for page table levels
Date: Thu, 15 Oct 2015 10:35:38 +0100 [thread overview]
Message-ID: <561F736A.4000308@arm.com> (raw)
In-Reply-To: <20151014170759.GE5041@leverpostej>
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
next prev parent reply other threads:[~2015-10-15 9:35 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-14 11:20 [PATCHv3 00/11] arm64: 16K translation granule support Suzuki K. Poulose
2015-10-14 11:20 ` [PATCHv3 01/11] arm64: Move swapper pagetable definitions Suzuki K. Poulose
2015-10-14 11:42 ` Mark Rutland
2015-10-14 12:41 ` Suzuki K. Poulose
2015-10-14 11:20 ` [PATCHv3 02/11] arm64: Handle section maps for swapper/idmap Suzuki K. Poulose
2015-10-14 12:06 ` Mark Rutland
2015-10-14 13:21 ` Suzuki K. Poulose
2015-10-14 14:51 ` Mark Rutland
2015-10-14 15:08 ` Suzuki K. Poulose
2015-10-14 15:14 ` Mark Rutland
2015-10-14 11:20 ` [PATCHv3 03/11] arm64: Introduce helpers for page table levels Suzuki K. Poulose
2015-10-14 17:07 ` Mark Rutland
2015-10-15 9:35 ` Suzuki K. Poulose [this message]
2015-10-15 10:37 ` Mark Rutland
2015-10-15 11:40 ` Christoffer Dall
2015-10-15 11:37 ` Christoffer Dall
2015-10-15 12:44 ` Mark Rutland
2015-10-15 13:14 ` Suzuki K. Poulose
2015-10-15 13:30 ` Christoffer Dall
2015-10-15 13:48 ` Suzuki K. Poulose
2015-10-15 14:15 ` Christoffer Dall
2015-10-14 11:20 ` [PATCHv3 04/11] arm64: Calculate size for idmap_pg_dir at compile time Suzuki K. Poulose
2015-10-14 17:12 ` Mark Rutland
2015-10-14 11:20 ` [PATCHv3 05/11] arm64: Handle 4 level page table for swapper Suzuki K. Poulose
2015-10-14 17:15 ` Mark Rutland
2015-10-14 11:20 ` [PATCHv3 06/11] arm64: Clean config usages for page size Suzuki K. Poulose
2015-10-14 11:20 ` [PATCHv3 07/11] arm64: Kconfig: Fix help text about AArch32 support with 64K pages Suzuki K. Poulose
2015-10-14 17:16 ` Mark Rutland
2015-10-14 11:20 ` [PATCHv3 08/11] arm64: Check for selected granule support Suzuki K. Poulose
2015-10-14 17:24 ` Mark Rutland
2015-10-14 17:32 ` Mark Rutland
2015-10-15 9:45 ` Suzuki K. Poulose
2015-10-15 10:39 ` Mark Rutland
2015-10-14 21:13 ` Jeremy Linton
2015-10-15 9:48 ` Suzuki K. Poulose
2015-10-15 10:45 ` Mark Rutland
2015-10-15 11:25 ` Suzuki K. Poulose
2015-10-15 12:37 ` Mark Rutland
2015-10-15 12:58 ` Suzuki K. Poulose
2015-10-16 8:03 ` Ard Biesheuvel
2015-10-15 14:47 ` Jeremy Linton
2015-10-15 15:02 ` Suzuki K. Poulose
2015-10-15 15:11 ` Mark Rutland
2015-10-16 8:11 ` Ard Biesheuvel
2015-10-14 11:20 ` [PATCHv3 09/11] arm64: Add page size to the kernel image header Suzuki K. Poulose
2015-10-14 17:27 ` Mark Rutland
2015-10-15 9:19 ` Suzuki K. Poulose
2015-10-14 11:20 ` [PATCHv3 10/11] arm64: Add 16K page size support Suzuki K. Poulose
2015-10-14 15:40 ` Jeremy Linton
2015-10-14 15:53 ` Suzuki K. Poulose
2015-10-15 14:06 ` Mark Rutland
2015-10-15 14:48 ` Suzuki K. Poulose
2015-10-15 15:36 ` Steve Capper
2015-10-15 15:48 ` Suzuki K. Poulose
2015-10-14 11:20 ` [PATCHv3 11/11] arm64: 36 bit VA Suzuki K. Poulose
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=561F736A.4000308@arm.com \
--to=suzuki.poulose@arm$(echo .)com \
--cc=linux-arm-kernel@lists$(echo .)infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox