Re: [PATCH] ARM: decompressor: reset ttbcr fields to use TTBR0 on ARMv7

From: Russell King - ARM Linux
Date: Mon Sep 12 2016 - 06:26:18 EST


On Mon, Sep 12, 2016 at 12:27:00PM +0530, Srinivas Ramana wrote:
> If the bootloader uses the long descriptor format and jumps to
> kernel decompressor code, TTBCR may not be in a right state.
> Before enabling the MMU, it is required to clear the TTBCR.PD0
> field to use TTBR0 for translation table walks.
>
> The 'commit dbece45894d3a ("ARM: 7501/1: decompressor:
> reset ttbcr for VMSA ARMv7 cores")' does the reset of TTBCR.N, but

There's no need to single-quote the commit part of this sentence.

> doesn't consider all the bits for the size of TTBCR.N.
>
> Clear TTBCR.PD0 field and reset all the three bits of TTBCR.N to
> indicate the use of TTBR0 and the correct base address width.
>

Adding a properly formatted Fixes: line here would be a bonus if we
need to backport it to stable trees.

> Signed-off-by: Srinivas Ramana <sramana@xxxxxxxxxxxxxx>
> ---
> arch/arm/boot/compressed/head.S | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index af11c2f8f3b7..fc6d541549a2 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -779,7 +779,7 @@ __armv7_mmu_cache_on:
> orrne r0, r0, #1 @ MMU enabled
> movne r1, #0xfffffffd @ domain 0 = client
> bic r6, r6, #1 << 31 @ 32-bit translation system
> - bic r6, r6, #3 << 0 @ use only ttbr0
> + bic r6, r6, #(7 << 0) | (1 << 4) @ use only ttbr0

Provided this is correct (I've not checked, I'd like an ack from one of
the ARM people, and I'd prefer to see a tested-by as well), and the above
points are addressed, it can be dropped into the patch system. Thanks.

--
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.