ARM: ARMv7-M: Add support for exception handling

This patch implements the exception handling for the ARMv7-M
architecture (pretty different from the A or R profiles).

It bases on work done earlier by Catalin for 2.6.33 but was nearly
completely rewritten to use a pt_regs layout compatible to the A
profile.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Jonathan Austin <jonathan.austin@arm.com>
Tested-by: Jonathan Austin <jonathan.austin@arm.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

1 files changed, 124 insertions, 0 deletions

diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index 9a8531e..b02e139 100644
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S
@@ -5,6 +5,7 @@
 #include <asm/asm-offsets.h>
 #include <asm/errno.h>
 #include <asm/thread_info.h>
+#include <asm/v7m.h>
 
 @ Bad Abort numbers
 @ -----------------
@@ -44,6 +45,116 @@
 #endif
 	.endm
 
+#ifdef CONFIG_CPU_V7M
+/*
+ * ARMv7-M exception entry/exit macros.
+ *
+ * xPSR, ReturnAddress(), LR (R14), R12, R3, R2, R1, and R0 are
+ * automatically saved on the current stack (32 words) before
+ * switching to the exception stack (SP_main).
+ *
+ * If exception is taken while in user mode, SP_main is
+ * empty. Otherwise, SP_main is aligned to 64 bit automatically
+ * (CCR.STKALIGN set).
+ *
+ * Linux assumes that the interrupts are disabled when entering an
+ * exception handler and it may BUG if this is not the case. Interrupts
+ * are disabled during entry and reenabled in the exit macro.
+ *
+ * v7m_exception_slow_exit is used when returning from SVC or PendSV.
+ * When returning to kernel mode, we don't return from exception.
+ */
+	.macro	v7m_exception_entry
+	@ determine the location of the registers saved by the core during
+	@ exception entry. Depending on the mode the cpu was in when the
+	@ exception happend that is either on the main or the process stack.
+	@ Bit 2 of EXC_RETURN stored in the lr register specifies which stack
+	@ was used.
+	tst	lr, #EXC_RET_STACK_MASK
+	mrsne	r12, psp
+	moveq	r12, sp
+
+	@ we cannot rely on r0-r3 and r12 matching the value saved in the
+	@ exception frame because of tail-chaining. So these have to be
+	@ reloaded.
+	ldmia	r12!, {r0-r3}
+
+	@ Linux expects to have irqs off. Do it here before taking stack space
+	cpsid	i
+
+	sub	sp, #S_FRAME_SIZE-S_IP
+	stmdb	sp!, {r0-r11}
+
+	@ load saved r12, lr, return address and xPSR.
+	@ r0-r7 are used for signals and never touched from now on. Clobbering
+	@ r8-r12 is OK.
+	mov	r9, r12
+	ldmia	r9!, {r8, r10-r12}
+
+	@ calculate the original stack pointer value.
+	@ r9 currently points to the memory location just above the auto saved
+	@ xPSR.
+	@ The cpu might automatically 8-byte align the stack. Bit 9
+	@ of the saved xPSR specifies if stack aligning took place. In this case
+	@ another 32-bit value is included in the stack.
+
+	tst	r12, V7M_xPSR_FRAMEPTRALIGN
+	addne	r9, r9, #4
+
+	@ store saved r12 using str to have a register to hold the base for stm
+	str	r8, [sp, #S_IP]
+	add	r8, sp, #S_SP
+	@ store r13-r15, xPSR
+	stmia	r8!, {r9-r12}
+	@ store old_r0
+	str	r0, [r8]
+	.endm
+
+        /*
+	 * PENDSV and SVCALL are configured to have the same exception
+	 * priorities. As a kernel thread runs at SVCALL execution priority it
+	 * can never be preempted and so we will never have to return to a
+	 * kernel thread here.
+         */
+	.macro	v7m_exception_slow_exit ret_r0
+	cpsid	i
+	ldr	lr, =EXC_RET_THREADMODE_PROCESSSTACK
+
+	@ read original r12, sp, lr, pc and xPSR
+	add	r12, sp, #S_IP
+	ldmia	r12, {r1-r5}
+
+	@ an exception frame is always 8-byte aligned. To tell the hardware if
+	@ the sp to be restored is aligned or not set bit 9 of the saved xPSR
+	@ accordingly.
+	tst	r2, #4
+	subne	r2, r2, #4
+	orrne	r5, V7M_xPSR_FRAMEPTRALIGN
+	biceq	r5, V7M_xPSR_FRAMEPTRALIGN
+
+	@ write basic exception frame
+	stmdb	r2!, {r1, r3-r5}
+	ldmia	sp, {r1, r3-r5}
+	.if	\ret_r0
+	stmdb	r2!, {r0, r3-r5}
+	.else
+	stmdb	r2!, {r1, r3-r5}
+	.endif
+
+	@ restore process sp
+	msr	psp, r2
+
+	@ restore original r4-r11
+	ldmia	sp!, {r0-r11}
+
+	@ restore main sp
+	add	sp, sp, #S_FRAME_SIZE-S_IP
+
+	cpsie	i
+	bx	lr
+	.endm
+#endif	/* CONFIG_CPU_V7M */
+
 	@
 	@ Store/load the USER SP and LR registers by switching to the SYS
 	@ mode. Useful in Thumb-2 mode where "stm/ldm rd, {sp, lr}^" is not
@@ -131,6 +242,18 @@
 	rfeia	sp!
 	.endm
 
+#ifdef CONFIG_CPU_V7M
+	/*
+	 * Note we don't need to do clrex here as clearing the local monitor is
+	 * part of each exception entry and exit sequence.
+	 */
+	.macro	restore_user_regs, fast = 0, offset = 0
+	.if	\offset
+	add	sp, #\offset
+	.endif
+	v7m_exception_slow_exit ret_r0 = \fast
+	.endm
+#else	/* ifdef CONFIG_CPU_V7M */
 	.macro	restore_user_regs, fast = 0, offset = 0
 	clrex					@ clear the exclusive monitor
 	mov	r2, sp
@@ -147,6 +270,7 @@
 	add	sp, sp, #S_FRAME_SIZE - S_SP
 	movs	pc, lr				@ return & move spsr_svc into cpsr
 	.endm
+#endif	/* ifdef CONFIG_CPU_V7M / else */
 
 	.macro	get_thread_info, rd
 	mov	\rd, sp