Blackfin: workaround anomaly 05000227

Workaround anomaly 05000227 by only using the scratch pad for stack when
absolutely necessary.  The core code which reprograms clocks really only
touches MMRs directly with constants.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>

arch/blackfin/mach-common/head.S

@@ -30,8 +30,6 @@ ENTRY(__init_clear_bss)
 	rts;
 ENDPROC(__init_clear_bss)
 
-#define INITIAL_STACK	(L1_SCRATCH_START + L1_SCRATCH_LENGTH - 12)
-
 ENTRY(__start)
 	/* R0: argument of command line string, passed from uboot, save it */
 	R7 = R0;
@@ -148,8 +146,8 @@ ENTRY(__start)
 #endif
 
 	/* Initialize stack pointer */
-	sp.l = lo(INITIAL_STACK);
-	sp.h = hi(INITIAL_STACK);
+	sp.l = _init_thread_union;
+	sp.h = _init_thread_union;
 	fp = sp;
 	usp = sp;
 
@@ -189,7 +187,15 @@ ENTRY(__start)
 	/* Put The Code for PLL Programming and SDRAM Programming in L1 ISRAM */
 	call _bfin_relocate_l1_mem;
 #ifdef CONFIG_BFIN_KERNEL_CLOCK
+	/* Only use on-chip scratch space for stack when absolutely required
+	 * to avoid Anomaly 05000227 ... we know the init_clocks() func only
+	 * uses L1 text and stack space and no other memory region.
+	 */
+# define KERNEL_CLOCK_STACK (L1_SCRATCH_START + L1_SCRATCH_LENGTH - 12)
+	sp.l = lo(KERNEL_CLOCK_STACK);
+	sp.h = hi(KERNEL_CLOCK_STACK);
 	call _init_clocks;
+	sp = usp;	/* usp hasnt been touched, so restore from there */
 #endif
 
 	/* This section keeps the processor in supervisor mode
@@ -243,9 +249,7 @@ ENTRY(_real_start)
 	call _cmdline_init;
 
 	/* Load the current thread pointer and stack */
-	sp.l = _init_thread_union;
-	sp.h = _init_thread_union;
-	p1 = THREAD_SIZE (z);
+	p1 = THREAD_SIZE + 4 (z);	/* +4 is for reti loading */
 	sp = sp + p1;
 	usp = sp;
 	fp = sp;