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ARM: 6663/1: make Thumb2 kernel entry point more similar to the ARM one
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Some installers would binary patch the kernel zImage to replace the
first few nops with custom instructions.  This breaks the Thumb2 kernel
as the mode switch is right at the beginning.  Let's move it towards the
end of the nop sequence instead.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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npitre authored and Russell King committed Feb 23, 2011
1 parent 74c25be commit b11fe38
Showing 1 changed file with 5 additions and 5 deletions.
10 changes: 5 additions & 5 deletions arch/arm/boot/compressed/head.S
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Expand Up @@ -128,14 +128,14 @@ wait: mrc p14, 0, pc, c0, c1, 0
.arm @ Always enter in ARM state
start:
.type start,#function
THUMB( adr r12, BSYM(1f) )
THUMB( bx r12 )
THUMB( .rept 6 )
ARM( .rept 8 )
.rept 7
mov r0, r0
.endr
ARM( mov r0, r0 )
ARM( b 1f )
THUMB( adr r12, BSYM(1f) )
THUMB( bx r12 )

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@kiban18

kiban18 Jun 28, 2013

First of all, let me share some history of the original purpose of repeat mov r0, r0 in ARM mode by Russell.
http://comments.gmane.org/gmane.linux.ports.arm.kernel/26690

Q1. Now I'm wondering what happens to this kernel if the boot loader Russell has mentioned calls this kernel at 0xNNNN8020. What value can be in r12 at 0xNNNN8020?
Here are the code compiled in thume-2 mode:
$ arm-linux-gnueabihf-objdump -d arch/arm/boot/compressed/head.o
00000000 :
0: e1a00000 nop ; (mov r0, r0)
4: e1a00000 nop ; (mov r0, r0)
8: e1a00000 nop ; (mov r0, r0)
c: e1a00000 nop ; (mov r0, r0)
10: e1a00000 nop ; (mov r0, r0)
14: e1a00000 nop ; (mov r0, r0)
18: e1a00000 nop ; (mov r0, r0)
1c: e28fc00d add ip, pc, #13
20: e12fff1c bx ip
24: 016f2818 .word 0x016f2818

Q2. How many nops can be replaced by the installers you've mentioned? (means boot loaders?) seven nops? or eight nops?

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@npitre

npitre via email Jun 28, 2013

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@kiban18

kiban18 Jun 28, 2013

Thanks for your answer, it is very helpful for me to understand the code.

b 1f
.word 0x016f2818 @ Magic numbers to help the loader
.word start @ absolute load/run zImage address
.word _edata @ zImage end address
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