[PATCH] ARM: decompressor: clear SCTLR.A bit for v7 cores

Dave Martin
2012-11-05T17:26:24Z

On Mon, Nov 05, 2012 at 11:13:51AM -0500, Nicolas Pitre wrote:
> On Mon, 5 Nov 2012, Russell King - ARM Linux wrote:
>
> > On Mon, Nov 05, 2012 at 01:02:55PM +0000, Dave Martin wrote:
> > > Why not allow unaligned accesses in the decompressor, though, both
> > > for v6 and v7?
> >
> > EXACTLY.
>
> I have no objections to that. In fact, I made a remark to this effect
> in my initial review of this patch. Whether or not gcc does take
> advantage of this hardware ability in the end is orthogonal.

For the sake of argument, here's how it might look.

Currently, I make no attempt to restore the original state of the U bit.
The A bit if forced later by the kernel during boot, after a short window
during which we should only run low-level arch code and therefore where
no unaligned accesses should happen.

Does anyone think these issues are likely to be important?

Cheers
---Dave

>From 160a5576b53264951ff8164775146b2d4feddecb Mon Sep 17 00:00:00 2001
From: Dave Martin <dave.martin@linaro.org>
Date: Mon, 5 Nov 2012 16:34:57 +0000
Subject: [PATCH] ARM: decompressor: Enable unaligned memory access for v6 and above

Modern GCC can generate code which makes use of the CPU's native
unaligned memory access capabilities. This is useful for the C
decompressor implementations used for unpacking compressed kernels.

This patch disables the alignment faults and enabled the v6
unaligned access on CPUs which support these features (i.e., v6 and
later), allowing full unaligned access support for C code in the
decompressor.

The decompressor C code must not be built to assume that unaligned
access works if support for v5 or older platforms is included in
the kernel.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
---
Note: I have only build-tested this so far.

arch/arm/boot/compressed/head.S | 12 +++++++++++-
1 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index 90275f0..cfbb4c9 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -652,6 +652,15 @@ __setup_mmu: sub r3, r4, #16384 @ Page directory size
mov pc, lr
ENDPROC(__setup_mmu)

+@ Enable unaligned access on v6, to allow better code generation
+@ for the decompressor C code:
+__armv6_mmu_cache_on:
+ mrc p15, 0, r0, c1, c0, 0 @ read SCTLR
+ bic r0, r0, #2 @ A (no unaligned access fault)
+ orr r0, r0, #1 << 22 @ U (v6 unaligned access model)
+ mcr p15, 0, r0, c1, c0, 0 @ write SCTLR
+ b __armv4_mmu_cache_on
+
__arm926ejs_mmu_cache_on:
#ifdef CONFIG_CPU_DCACHE_WRITETHROUGH
mov r0, #4 @ put dcache in WT mode
@@ -694,6 +703,7 @@ __armv7_mmu_cache_on:
bic r0, r0, #1 << 28 @ clear SCTLR.TRE
orr r0, r0, #0x5000 @ I-cache enable, RR cache replacement
orr r0, r0, #0x003c @ write buffer
+ bic r0, r0, #2 @ A (no unaligned access fault)
#ifdef CONFIG_MMU
#ifdef CONFIG_CPU_ENDIAN_BE8
orr r0, r0, #1 << 25 @ big-endian page tables
@@ -914,7 +924,7 @@ proc_types:

.word 0x0007b000 @ ARMv6
.word 0x000ff000
- W(b) __armv4_mmu_cache_on
+ W(b) __armv6_mmu_cache_on
W(b) __armv4_mmu_cache_off
W(b) __armv6_mmu_cache_flush

--
1.7.4.1