ARCv2: entry: comments about hardware auto-save on taken interrupts

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

diff --git a/arch/arc/include/asm/entry-arcv2.h b/arch/arc/include/asm/entry-arcv2.h
index 225e7df..1c3520d 100644 (file)
--- 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