MIPS: fix ROM exception vectors

When booting from ROM, early exceptions can't be handled
properly. Instead of busy-looping give the developer the
possibilty to examine the situation. Invoke an UHI
exception operation which can be read as unhandled exception
by a hardware debugger if one is attached. If the debugger
doesn't support UHI, the exception is read as unexpected
breakpoint.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

diff --git a/arch/mips/cpu/start.S b/arch/mips/cpu/start.S
index 108d2df..2397b6c 100644 (file)
--- a/arch/mips/cpu/start.S
+++ b/arch/mips/cpu/start.S
@@ -52,6 +52,14 @@
 
        .set noreorder
 
+       .macro uhi_mips_exception
+       move    k0, t9          # preserve t9 in k0
+       move    k1, a0          # preserve a0 in k1
+       li      t9, 15          # UHI exception operation
+       li      a0, 0           # Use hard register context
+       sdbbp   1               # Invoke UHI operation
+       .endm
+
 ENTRY(_start)
        /* U-Boot entry point */
        b       reset
@@ -79,30 +87,30 @@ ENTRY(_start)
 #endif
 
 #if defined(CONFIG_ROM_EXCEPTION_VECTORS)
+       /*
+        * Exception vector entry points. When running from ROM, an exception
+        * cannot be handled. Halt execution and transfer control to debugger,
+        * if one is attached.
+        */
        .org 0x200
        /* TLB refill, 32 bit task */
-1:     b       1b
-        nop
+       uhi_mips_exception
 
        .org 0x280
        /* XTLB refill, 64 bit task */
-1:     b       1b
-        nop
+       uhi_mips_exception
 
        .org 0x300
        /* Cache error exception */
-1:     b       1b
-        nop
+       uhi_mips_exception
 
        .org 0x380
        /* General exception */
-1:     b       1b
-        nop
+       uhi_mips_exception
 
        .org 0x400
        /* Catch interrupt exceptions */
-1:     b       1b
-        nop
+       uhi_mips_exception
 
        .org 0x480
        /* EJTAG debug exception */
@@ -222,6 +230,7 @@ reset:
 
        move    a0, zero                # a0 <-- boot_flags = 0
        PTR_LA  t9, board_init_f
+
        jr      t9
         move   ra, zero