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ARM: 6999/1: head, zImage: Always Enter the kernel in ARM state

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Currently, the documented kernel entry requirements are not
explicit about whether the kernel should be entered in ARM or
Thumb, leading to an ambiguitity about how to enter Thumb-2
kernels.  As a result, the kernel is reliant on the zImage
decompressor to enter the kernel proper in the correct instruction
set state.

This patch changes the boot entry protocol for head.S and Image to
be the same as for zImage: in all cases, the kernel is now entered
in ARM.

Documentation/arm/Booting is updated to reflect this new policy.

A different rule will be needed for Cortex-M class CPUs as and when
support for those lands in mainline, since these CPUs don't support
the ARM instruction set at all: a note is added to the effect that
the kernel must be entered in Thumb on such systems.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Dave Martin 2011-07-13 15:53:30 +01:00 committed by Russell King
parent c7e89b16eb
commit 540b573875
4 changed files with 23 additions and 1 deletions
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5
Documentation/arm/Booting
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@ -164,3 +164,8 @@ In either case, the following conditions must be met:
- The boot loader is expected to call the kernel image by jumping
directly to the first instruction of the kernel image.
On CPUs supporting the ARM instruction set, the entry must be
made in ARM state, even for a Thumb-2 kernel.
On CPUs supporting only the Thumb instruction set such as
Cortex-M class CPUs, the entry must be made in Thumb state.

3
arch/arm/boot/compressed/head.S
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@ -353,7 +353,8 @@ not_relocated: mov r0, #0
mov r0, #0 @ must be zero
mov r1, r7 @ restore architecture number
mov r2, r8 @ restore atags pointer
mov pc, r4 @ call kernel
ARM( mov pc, r4 ) @ call kernel
THUMB( bx r4 ) @ entry point is always ARM
.align 2
.type LC0, #object

8
arch/arm/kernel/head-nommu.S
View File

@ -32,8 +32,16 @@
* numbers for r1.
*
*/
.arm
__HEAD
ENTRY(stext)
THUMB( adr r9, BSYM(1f) ) @ Kernel is always entered in ARM.
THUMB( bx r9 ) @ If this is a Thumb-2 kernel,
THUMB( .thumb ) @ switch to Thumb now.
THUMB(1: )
setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode
@ and irqs disabled
#ifndef CONFIG_CPU_CP15

8
arch/arm/kernel/head.S
View File

@ -71,8 +71,16 @@
* crap here - that's what the boot loader (or in extreme, well justified
* circumstances, zImage) is for.
*/
.arm
__HEAD
ENTRY(stext)
THUMB( adr r9, BSYM(1f) ) @ Kernel is always entered in ARM.
THUMB( bx r9 ) @ If this is a Thumb-2 kernel,
THUMB( .thumb ) @ switch to Thumb now.
THUMB(1: )
setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode
@ and irqs disabled
mrc p15, 0, r9, c0, c0 @ get processor id