[PATCH 2/6] ARM: gic: fix big endian support

[dave.martin@linaro.org (Dave Martin)]

On Tue, Nov 22, 2011 at 09:47:24AM +0800, Bi Junxiao wrote:
> on 11/22/2011 03:11 AM Nicolas Pitre wrote:
> >On Tue, 15 Nov 2011, Junxiao Bi wrote:
> >
> >>The irq state from gic is in little-endian mode. We need do endian
> >>conversion for it in big-endian machine. Usually byte swapping of
> >>32-bits word needs 4 assembler instructions for arm machine which
> >>arch<  6. But since the high order half word of the irq state is
> >>zero, there is a way that can use lesser instrunctions to improve
> >>the performance since we only need a bytes swaping of half word.
> >>That's what swab16_low_half and swab16_high_half do.
> >>
> >>Signed-off-by: Junxiao Bi<junxiao.bi@windriver.com>
> >Comments below.
> >
> >>---
> >>  arch/arm/include/asm/assembler.h                |   27 +++++++++++++++++++++++
> >>  arch/arm/include/asm/hardware/entry-macro-gic.S |    7 ++++++
> >>  2 files changed, 34 insertions(+), 0 deletions(-)
> >>
> >>diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> >>index 29035e8..3ddee22 100644
> >>--- a/arch/arm/include/asm/assembler.h
> >>+++ b/arch/arm/include/asm/assembler.h
> >>@@ -302,4 +302,31 @@
> >>  	.size \name , . - \name
> >>  	.endm
> >>
> >>+/*
> >>+ * swab16_low_half is used to swap the bytes order of the low order
> >>+ * half word, it assumes that the high order half word is zero.
> >>+ * swab16_high_half is used to swap the bytes order of the high order
> >>+ * half word, it assumes that the low order half word is zero.
> >>+ */
> >>+#if __LINUX_ARM_ARCH__>= 6
> >>+	.macro swab16_low_half, reg
> >>+	rev \reg, \reg
> >>+	.endm
> >>+
> >>+	.macro swab16_high_half, reg
> >>+	rev \reg, \reg
> >>+	.endm
> >>+#else
> >>+	.macro swab16_low_half, reg
> >>+	mov \reg, \reg, ror #8
> >>+	orr \reg, \reg, \reg, lsr #16
> >>+	bic \reg, \reg, \reg, lsl #16
> >>+	.endm

Your v6 and <v6 implementations don't seem to produce the same result
for swab16_low_half.

If reg=0x0000AABB  on >=v6 you transform it to 0xBBAA0000, but on <v6
you translate it to 0x0000BBAA.

Perhaps you meant to use the rev16 instruction instead of rev?

> >I fail to see how this macro would be generally useful.  Presuming that
> >the top or bottom half has to be zero is not particularly practical.
> We can use it to swap bytes order of a 16-bits number in 32-bits
> register in which case the top or bottom half will be zero. For the
> \irqstat register, as the original implementation assumes its top
> half is zero, so I make the same assumption.
> >>+	.macro swab16_high_half, reg
> >>+	swab16_low_half \reg
> >>+	mov \reg, \reg, lsl #16
> >>+	.endm
> >And this doesn't end up cheaper than a plain swab32.
> This macro has an advantage that it only needs one register. For
> swab32, it will need another temporary register besides the target
> register.
> >Furthermore, you're using those macros 1) in the gic code, and 2) onli
> >in the BE8 case.  Both of those conditions are only possible on ARMv6
> >and above anyway, so I'd simply open code a rev instruction inline.
> This is not be8 specific. It will be needed for both be8 and be32.
> So the machine with arch < 6 also needs to be supported.

I also thought the GIC doesn't get used with <v6?  I thought the GIC was
too new for that, but I'm not an expert on it.

Cheers
---Dave