[PATCH v2 3/5] ARM: use generic unaligned.h

[Nicolas Pitre]

On Wed, 10 Oct 2012, Rob Herring wrote:

> On 10/08/2012 11:01 PM, Nicolas Pitre wrote:
> > On Mon, 8 Oct 2012, Rob Herring wrote:
> > 
> >> On 10/08/2012 06:28 PM, Shawn Guo wrote:
> >>> On Mon, Oct 08, 2012 at 03:34:57PM -0500, Rob Herring wrote:
> >>>> On 10/08/2012 11:43 AM, Shawn Guo wrote:
> >>>>> This patch has been merged into mainline as commit below.
> >>>>>
> >>>>>  d25c881 ARM: 7493/1: use generic unaligned.h
> >>>>>
> >>>>> It introduces a regression for me.  Check out the commit on mainline,
> >>>>> build a v7 only kernel (imx5/6) with imx_v6_v7_defconfig, the kernel
> >>>>> halts in decompressor.  But v6/v7 kernel (imx3/5/6) works fine.  The
> >>>>> kernel built on the parent commit below works all fine.
> >>>>
> >>>> It actually fails in the decompressor or that's the last output you get?
> >>>
> >>> I think it fails in the decompressor, because what I see is 
> >>>
> >>> Uncompressing Linux...
> >>>
> >>> not
> >>>
> >>> Uncompressing Linux... done, booting the kernel.
> >>>
> >>>> I compared the decompressor disassembly of both cases and get the same
> >>>> number of ldrb/strb instructions, so I don't think it is directly
> >>>> related to alignment.
> >>>>
> >>>> I tried the XY decompressor as that is one difference, but that works
> >>>> fine for me on highbank.
> >>>>
> >>>> Does it work with an empty uncompress.h functions? That should be the
> >>>> only difference in our decompressor code.
> >>>
> >>> No, empty putc() in uncompress.h does not help.
> >>>
> >>> New finding is that it only fails with LZO decompressor while the other
> >>> 3 Gzip, LZMA and XZ all work good.  We happen to have LZO as the default
> >>> one in imx_v6_v7_defconfig.
> >>
> >> I must have had an old config with XZ. LZO fails because of this:
> >>  
> >> lib/decompress_unlzo.c: version = get_unaligned_be16(parse);
> >> lib/decompress_unlzo.c: if (get_unaligned_be32(parse) & HEADER_HAS_FILTER)
> >> lib/decompress_unlzo.c:         dst_len = get_unaligned_be32(in_buf);
> >> lib/decompress_unlzo.c:         src_len = get_unaligned_be32(in_buf);
> >>
> >> This was what I was afraid of. The decompressor runs with the sysctrl
> >> register A bit in whatever state the bootloader left it in. In the case
> >> of u-boot it is set, and the maintainers are pretty set on not allowing
> >> unaligned accesses if you've seen the recent discussion.
> > 
> > This is not an u-Boot issue.  The kernel code expects misaligned 
> > accesses to be handled by the hardware on ARMv6+ now.  So it better 
> > enforce proper A bit state itself.
> > 
> >> This should fix things.
> >>
> >> Rob
> >>
> >> 8<---------------------------------------------------------------------
> >>
> >> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> >> index bc67cbf..1f87d22 100644
> >> --- a/arch/arm/boot/compressed/head.S
> >> +++ b/arch/arm/boot/compressed/head.S
> >> @@ -654,6 +654,7 @@ __armv7_mmu_cache_on:
> >>  #endif
> >>                 mrc     p15, 0, r0, c1, c0, 0   @ read control reg
> >>                 bic     r0, r0, #1 << 28        @ clear SCTLR.TRE
> >> +               bic     r0, r0, #1 << 1         @ clear SCTLR.A
> >>                 orr     r0, r0, #0x5000         @ I-cache enable, RR cache replacement
> >>                 orr     r0, r0, #0x003c         @ write buffer
> >>  #ifdef CONFIG_MMU
> >>
> > 
> > That's OK for ARMv7, but in the ARMv6 case we use __armv4_mmu_cache_on.  
> > So this patch is incomplete.
> 
> It is still optional on v6 and the compiler will not emit unaligned
> accesses for v6 (which is why it worked when v6 platforms were enabled).

Hmmm, right. However if you look at commit 8428e84d42, the A bit is 
cleared whenever we compile for ARMv6 or higher.  Of course, the 
decompressor doesn't have to deal with unknown user space binaries and 
in that case we might trust that the compiler will never emit known to 
be unaligned loads.

In that case...

Acked-by: Nicolas Pitre <nico at linaro.org>

> That does raise a bigger question. If we're doing combined v6 and v7
> builds, do we want this to require unaligned accesses are enabled and if
> so, can we force that on in the compiler?

When we compile ARMv6 and ARMv7 targets together, the compiler is told 
to compile for ARMv6.  We know that unaligned accesses will be done 
usingLDRB/STRB in that case.


Nicolas