nds32: start_irq_handler() use system stack

[symptom]
The 'vboot_hash_start()' always return busy error and variable 'in_progress'
got a strange value(should be 0 or 1).

'start_irq_handler()' causes scratchpad overflow in first context switch.
It must be called after SP switch to system stack in ISR.

NOTE:
The scratchpad is still also need more size even if
'start_irq_handler()' is using system stack. following is detail:

1. uint32_t scratchpad[19] 0x81d34 ~ 0x81d7f
[__task_start:]
2. /* put the dummy stack pointer at the top of the stack in scratchpad */
   addi $sp, $r3, 4 * 18
   -> SP 0x81d7c
3. syscall push return address (-4)
   -> SP 0x81d78
[ISR:]
4. push r15, fp, lp, and sp (-0x10)
   -> SP 0x81d68
5. push r0 ~ r5 (-0x18)
   -> SP 0x81d50
[__switch_task:]
6. /* save ipsw, ipc, r6, r7, r8, r9, r10 on the current process stack */ (-0x1C)
-> SP 0x81d34

Signed-off-by: Dino Li <dino.li@ite.com.tw>

BRANCH=none
BUG=none
TEST=1. hash done.
     2. console command 'taskinfo'.
     3. the scratchpad does not overflow after first context switch.

Change-Id: If5d89ff5c945a777010492fcfb54bf41f434ed69
Reviewed-on: https://chromium-review.googlesource.com/317468
Commit-Ready: Dino Li <dino.li@ite.com.tw>
Tested-by: Dino Li <dino.li@ite.com.tw>
Reviewed-by: Randall Spangler <rspangler@chromium.org>

core/nds32/init.S

@@ -20,13 +20,13 @@ __entry_\()\name:
 	smw.adm $r15, [$sp], $r15, 0xb
 	/* r0-r5 are caller saved */
 	smw.adm $r0,  [$sp], $r5, 0
-	/* isr entry */
-	jal start_irq_handler
 	/* switch to system stack if we are called from process stack */
 	la $r3, stack_end
 	mov55 $fp, $sp
 	slt45 $r3, $sp /* if sp > end of system stack, then r15 = 1 and */
 	cmovn $sp, $r3, $r15 /* point sp to the top of the system stack */
+	/* isr entry */
+	jal start_irq_handler
 	/* C routine handler */
 	jal \name\()_handler
 	/* check whether we need to change the scheduled task */

core/nds32/switch.S

@@ -89,7 +89,7 @@ __task_start:
 	movi55 $r0, 0    /* syscall 1st parameter : de-schedule nothing */
 
 	/* put the dummy stack pointer at the top of the stack in scratchpad */
-	addi $sp, $r3, 4 * 16
+	addi $sp, $r3, 4 * 18
 	/* we are ready to re-schedule */
 	swi.gp  $r4, [ + need_resched]
 

core/nds32/task.c

@@ -145,9 +145,9 @@ uint8_t task_stacks[0
 
 /* Reserve space to discard context on first context switch. */
 #ifdef CONFIG_FPU
-uint32_t scratchpad[17+18];
+uint32_t scratchpad[19+18];
 #else
-uint32_t scratchpad[17];
+uint32_t scratchpad[19];
 #endif
 
 task_ *current_task = (task_ *)scratchpad;