Blackfin: use scratch pad for exception stack

If the memory layout pushes the stack out of the default DCPLB coverage,
the exception handler may trigger a double fault by trying to push onto
the uncovered stack.  So handle the exception stack similar to the kernel
by using the top of the scratch pad SRAM.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>

cpu/blackfin/interrupt.S

@@ -5,6 +5,7 @@
  * Licensed under the GPL-2 or later.
  */
 
+#include <config.h>
 #include <asm/blackfin.h>
 #include <asm/entry.h>
 
@@ -12,12 +13,16 @@
 
 /* default entry point for exceptions */
 ENTRY(_trap)
+	CONFIG_BFIN_SCRATCH_REG = sp;
+	sp.l = LO(L1_SRAM_SCRATCH_END - 20);
+	sp.h = HI(L1_SRAM_SCRATCH_END - 20);
 	SAVE_ALL_SYS
 	r0 = sp;	/* stack frame pt_regs pointer argument ==> r0 */
 	sp += -12;
 	call _trap_c;
 	sp += 12;
 	RESTORE_ALL_SYS
+	sp = CONFIG_BFIN_SCRATCH_REG;
 	rtx;
 ENDPROC(_trap)
 

include/asm-blackfin/config.h

@@ -14,6 +14,10 @@
 # error CONFIG_BFIN_CPU: your board config needs to define this
 #endif
 
+#ifndef CONFIG_BFIN_SCRATCH_REG
+# define CONFIG_BFIN_SCRATCH_REG retn
+#endif
+
 /* Make sure the structure is properly aligned */
 #if ((CONFIG_SYS_GBL_DATA_ADDR & -4) != CONFIG_SYS_GBL_DATA_ADDR)
 # error CONFIG_SYS_GBL_DATA_ADDR: must be 4 byte aligned