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Subject[PATCH 1/5] ARCv2: entry: comments about hardware auto-save on taken interrupts
Date
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
---
arch/arc/include/asm/entry-arcv2.h | 78 ++++++++++++++++++++++++++++++--------
1 file changed, 62 insertions(+), 16 deletions(-)

diff --git a/arch/arc/include/asm/entry-arcv2.h b/arch/arc/include/asm/entry-arcv2.h
index 225e7df2d8ed..1c3520d1fa42 100644
--- a/arch/arc/include/asm/entry-arcv2.h
+++ b/arch/arc/include/asm/entry-arcv2.h
@@ -7,15 +7,54 @@
#include <asm/irqflags-arcv2.h>
#include <asm/thread_info.h> /* For THREAD_SIZE */

+/*
+ * Interrupt/Exception stack layout (pt_regs) for ARCv2
+ * (End of struct aligned to end of page [unless nested])
+ *
+ * INTERRUPT EXCEPTION
+ *
+ * manual --------------------- manual
+ * | orig_r0 |
+ * | event/ECR |
+ * | bta |
+ * | user_r25 |
+ * | gp |
+ * | fp |
+ * | sp |
+ * | r12 |
+ * | r30 |
+ * | r58 |
+ * | r59 |
+ * hw autosave ---------------------
+ * optional | r0 |
+ * | r1 |
+ * ~ ~
+ * | r9 |
+ * | r10 |
+ * | r11 |
+ * | blink |
+ * | lpe |
+ * | lps |
+ * | lpc |
+ * | ei base |
+ * | ldi base |
+ * | jli base |
+ * ---------------------
+ * hw autosave | pc / eret |
+ * mandatory | stat32 / erstatus |
+ * ---------------------
+ */
+
/*------------------------------------------------------------------------*/
.macro INTERRUPT_PROLOGUE called_from
-
- ; Before jumping to Interrupt Vector, hardware micro-ops did following:
+ ; (A) Before jumping to Interrupt Vector, hardware micro-ops did following:
; 1. SP auto-switched to kernel mode stack
- ; 2. STATUS32.Z flag set to U mode at time of interrupt (U:1, K:0)
- ; 3. Auto saved: r0-r11, blink, LPE,LPS,LPC, JLI,LDI,EI, PC, STAT32
+ ; 2. STATUS32.Z flag set if in U mode at time of interrupt (U:1,K:0)
+ ; 3. Auto save: (mandatory) Push PC and STAT32 on stack
+ ; hardware does even if CONFIG_ARC_IRQ_NO_AUTOSAVE
+ ; 4. Auto save: (optional) r0-r11, blink, LPE,LPS,LPC, JLI,LDI,EI
;
- ; Now manually save: r12, sp, fp, gp, r25
+ ; (B) Manually saved some regs: r12,r25,r30, sp,fp,gp, ACCL pair

#ifdef CONFIG_ARC_IRQ_NO_AUTOSAVE
.ifnc \called_from, exception
@@ -57,14 +96,17 @@
; - U mode: retrieve it from AUX_USER_SP
; - K mode: add the offset from current SP where H/w starts auto push
;
- ; Utilize the fact that Z bit is set if Intr taken in U mode
+ ; 1. Utilize the fact that Z bit is set if Intr taken in U mode
+ ; 2. Upon entry SP is always saved (for any inspection, unwinding etc),
+ ; but on return, restored only if U mode
+
mov.nz r9, sp
- add.nz r9, r9, SZ_PT_REGS - PT_sp - 4
+ add.nz r9, r9, SZ_PT_REGS - PT_sp - 4 ; K mode SP
bnz 1f

- lr r9, [AUX_USER_SP]
+ lr r9, [AUX_USER_SP] ; U mode SP
1:
- PUSH r9 ; SP
+ PUSH r9 ; SP (pt_regs->sp)

PUSH fp
PUSH gp
@@ -85,6 +127,8 @@
/*------------------------------------------------------------------------*/
.macro INTERRUPT_EPILOGUE called_from

+ ; INPUT: r0 has STAT32 of calling context
+ ; INPUT: Z flag set if returning to K mode
.ifnc \called_from, exception
add sp, sp, 12 ; skip BTA/ECR/orig_r0 placeholderss
.endif
@@ -98,9 +142,10 @@
POP gp
POP fp

- ; Don't touch AUX_USER_SP if returning to K mode (Z bit set)
- ; (Z bit set on K mode is inverse of INTERRUPT_PROLOGUE)
- add.z sp, sp, 4
+ ; Restore SP (into AUX_USER_SP) only if returning to U mode
+ ; - for K mode, it will be implicitly restored as stack is unwound
+ ; - Z flag set on K is inverse of what hardware does on interrupt entry
+ ; but that doesn't really matter
bz 1f

POPAX AUX_USER_SP
@@ -145,11 +190,11 @@
/*------------------------------------------------------------------------*/
.macro EXCEPTION_PROLOGUE

- ; Before jumping to Exception Vector, hardware micro-ops did following:
+ ; (A) Before jumping to Exception Vector, hardware micro-ops did following:
; 1. SP auto-switched to kernel mode stack
- ; 2. STATUS32.Z flag set to U mode at time of interrupt (U:1,K:0)
+ ; 2. STATUS32.Z flag set if in U mode at time of exception (U:1,K:0)
;
- ; Now manually save the complete reg file
+ ; (B) Manually save the complete reg file below

PUSH r9 ; freeup a register: slot of erstatus

@@ -195,12 +240,13 @@
PUSHAX ecr ; r9 contains ECR, expected by EV_Trap

PUSH r0 ; orig_r0
+ ; OUTPUT: r9 has ECR
.endm

/*------------------------------------------------------------------------*/
.macro EXCEPTION_EPILOGUE

- ; Assumes r0 has PT_status32
+ ; INPUT: r0 has STAT32 of calling context
btst r0, STATUS_U_BIT ; Z flag set if K, used in INTERRUPT_EPILOGUE

add sp, sp, 8 ; orig_r0/ECR don't need restoring
--
2.7.4
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