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SubjectRe: [PATCH] Fix non-LPAE boot regression.
Hi Vasily,

On Sat, Aug 13, 2011 at 01:58:19PM +0100, Vasily Khoruzhick wrote:
> It was introduced by 407f8b4cb07cbc5c1c7cc386f231224e2524ccea
> ARM: LPAE: MMU setup for the 3-level page table format
>
> Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
> ---
> arch/arm/kernel/head.S | 4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> index 0bdafc4..5add5f5 100644
> --- a/arch/arm/kernel/head.S
> +++ b/arch/arm/kernel/head.S
> @@ -206,7 +206,7 @@ __create_page_tables:
> 1: orr r3, r7, r5, lsl #SECTION_SHIFT @ flags + kernel base
> str r3, [r4, r5, lsl #PMD_ORDER] @ identity mapping
> cmp r5, r6
> - addlo r5, r5, #SECTION_SHIFT >> 20 @ next section
> + addlo r5, r5, #1 @ next section
> blo 1b

That's correct.

> /*
> @@ -217,7 +217,7 @@ __create_page_tables:
> mov r3, r3, lsr #SECTION_SHIFT
> orr r3, r7, r3, lsl #SECTION_SHIFT
> add r0, r4, #(KERNEL_START & 0xff000000) >> (SECTION_SHIFT - PMD_ORDER)
> - str r3, [r0, #(KERNEL_START & 0x00e00000) >> (SECTION_SHIFT - PMD_ORDER)]!
> + str r3, [r0, #(KERNEL_START & 0x00f00000) >> (SECTION_SHIFT - PMD_ORDER)]!

The reason for this was that the sections are 2MB with LPAE and a page
table entry is 64-bit wide. We always shift that value by 18 but with
LPAE we don't want to write in the middle of a page table entry if
KERNEL_START is not 2MB aligned.

But if KERNEL_START is not 2MB aligned, I think we get the wrong
physical address by 1MB (with the classic page table format).

There are a few alternatives to fixing this:

1. Different KERNEL_START masking for classic or LPAE page tables.
2. Always force 2MB section and the code above moving the phys addr into
r3 would need to take this into account.

I would go for 1 with some shifting like below:

+ str r3, [r0, #((KERNEL_START & 0x00f00000) >> SECTION_SHIFT) << PMD_ORDER]!
This should give us 0x00f00000 with classic page tables and 0x00e00000
with LPAE.
Thanks.

--
Catalin


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