arch/tile: work around a hardware issue with the return-address stack

In certain circumstances we need to do a bunch of jump-and-link
instructions to fill the hardware return-address stack with nonzero values.
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/include/asm/traps.h

@@ -64,7 +64,11 @@ void do_breakpoint(struct pt_regs *, int fault_num);
 
 
 #ifdef __tilegx__
+/* kernel/single_step.c */
 void gx_singlestep_handle(struct pt_regs *, int fault_num);
+
+/* kernel/intvec_64.S */
+void fill_ra_stack(void);
 #endif
 
-#endif /* _ASM_TILE_SYSCALLS_H */
+#endif /* _ASM_TILE_TRAPS_H */

arch/tile/kernel/intvec_64.S

@@ -1156,6 +1156,18 @@ int_unalign:
 	push_extra_callee_saves r0
 	j       do_trap
 
+/* Fill the return address stack with nonzero entries. */
+STD_ENTRY(fill_ra_stack)
+	{
+	 move	r0, lr
+	 jal	1f
+	}
+1:	jal	2f
+2:	jal	3f
+3:	jal	4f
+4:	jrp	r0
+	STD_ENDPROC(fill_ra_stack)
+
 /* Include .intrpt1 array of interrupt vectors */
 	.section ".intrpt1", "ax"
 

arch/tile/kernel/traps.c

@@ -289,7 +289,10 @@ void __kprobes do_trap(struct pt_regs *regs, int fault_num,
 		address = regs->pc;
 		break;
 #ifdef __tilegx__
-	case INT_ILL_TRANS:
+	case INT_ILL_TRANS: {
+		/* Avoid a hardware erratum with the return address stack. */
+		fill_ra_stack();
+
 		signo = SIGSEGV;
 		code = SEGV_MAPERR;
 		if (reason & SPR_ILL_TRANS_REASON__I_STREAM_VA_RMASK)
@@ -297,6 +300,7 @@ void __kprobes do_trap(struct pt_regs *regs, int fault_num,
 		else
 			address = 0;  /* FIXME: GX: single-step for address */
 		break;
+	}
 #endif
 	default:
 		panic("Unexpected do_trap interrupt number %d", fault_num);