mips: Use a separate IRQ stack & optimize IRQ handling a little

Should prevent the IRQ-related stack overflows seen on MIPS targets.

Change-Id: I447336ef3fe37e11b3276a78ba220ce64c2f87f5

firmware/target/mips/ingenic_jz47xx/app.lds

@@ -83,6 +83,9 @@ SECTIONS
          stackbegin = .;
          . += 0x2000;
          stackend = .;
+         irqstackbegin = .;
+         . += 0x400;
+         irqstackend = .;
     } > IRAM
 
     .bss (NOLOAD):

firmware/target/mips/ingenic_jz47xx/boot.lds

@@ -87,5 +87,8 @@ SECTIONS
          stackbegin = .;
          . += 0x2000;
          stackend = .;
+         irqstackbegin = .;
+         . += 0x400;
+         irqstackend = .;
     } > IRAM
 }

firmware/target/mips/ingenic_jz47xx/crt0.S

@@ -164,6 +164,19 @@ _stack_loop:
     bne     t0, sp, _stack_loop
     sw      t2, -4(t0)
 
+    /*
+      ----------------------------------------------------
+       Set up interrupt stack
+      ----------------------------------------------------
+    */
+    la     k0, irqstackend
+    la     t0, irqstackbegin
+
+_irq_stack_loop:
+    addiu  t0, 4
+    bne    t0, k0, _irq_stack_loop
+    sw     t2, -4(t0)
+
     /*
       ----------------------------------------------------
        Jump to C code
@@ -200,7 +213,17 @@ _stack_loop:
 
     .section .vectors, "ax", %progbits
 real_exception_handler:
-    addiu   sp, -0x80
+
+    /* Store stack pointer */
+    move    k0, sp
+    /* jump to IRQ stack */
+    la      sp, irqstackend
+
+    /* Push crap on frame */
+    addiu   sp, -0x84
+    /* store current stack pointer */
+    sw      k0, 0x80(sp)
+
     sw      ra,    0(sp)
     sw      fp,    4(sp)
     sw      gp,    8(sp)
@@ -236,16 +259,14 @@ real_exception_handler:
     nop
     sw      k0, 0x74(sp)
     mfc0    k0, C0_STATUS
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
     sw      k0, 0x78(sp)
     mfc0    k0, C0_EPC
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
     sw      k0, 0x7C(sp)
 
     li      k1, M_CauseExcCode
@@ -264,15 +285,13 @@ _int:
 _exception:
     move    a0, sp
     mfc0    a1, C0_CAUSE
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
     mfc0    a2, C0_EPC
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
     jal     exception_handler
     nop
 
@@ -314,18 +333,15 @@ _exception_return:
     lw      k0, 0x78(sp)
     mtc0    k0, C0_STATUS
     nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
-    sll     zero, 1
     lw      k0, 0x7C(sp)
     mtc0    k0, C0_EPC
     nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    nop
-    sll     zero, 1
+    /* Restore previous stack pointer */
-    sll     zero, 1
+    lw      sp, 0x80(sp)
-    addiu   sp, 0x80
     eret
     nop
     .set reorder