4 files changed, 83 insertions, 3 deletions

diff --git a/arch/tile/include/asm/traps.h b/arch/tile/include/asm/traps.h
index 432a9c1..d06e35f 100644
--- a/arch/tile/include/asm/traps.h
+++ b/arch/tile/include/asm/traps.h
@@ -59,4 +59,8 @@ void do_hardwall_trap(struct pt_regs *, int fault_num);
 void do_breakpoint(struct pt_regs *, int fault_num);
 
 
+#ifdef __tilegx__
+void gx_singlestep_handle(struct pt_regs *, int fault_num);
+#endif
+
 #endif /* _ASM_TILE_SYSCALLS_H */
diff --git a/arch/tile/kernel/intvec_32.S b/arch/tile/kernel/intvec_32.S
index 206dc7e..f5821626 100644
--- a/arch/tile/kernel/intvec_32.S
+++ b/arch/tile/kernel/intvec_32.S
@@ -1472,7 +1472,12 @@ handle_ill:
 	lw      r26, r24
 	sw      r28, r26
 
-	/* Clear TIF_SINGLESTEP */
+	/*
+	 * Clear TIF_SINGLESTEP to prevent recursion if we execute an ill.
+	 * The normal non-arch flow redundantly clears TIF_SINGLESTEP, but we
+	 * need to clear it here and can't really impose on all other arches.
+	 * So what's another write between friends?
+	 */
 	GET_THREAD_INFO(r0)
 
 	addi    r1, r0, THREAD_INFO_FLAGS_OFFSET
diff --git a/arch/tile/kernel/single_step.c b/arch/tile/kernel/single_step.c
index 5ec4b9c..1eb3b39 100644
--- a/arch/tile/kernel/single_step.c
+++ b/arch/tile/kernel/single_step.c
@@ -15,7 +15,7 @@
  * Derived from iLib's single-stepping code.
  */
 
-#ifndef __tilegx__   /* No support for single-step yet. */
+#ifndef __tilegx__   /* Hardware support for single step unavailable. */
 
 /* These functions are only used on the TILE platform */
 #include <linux/slab.h>
@@ -660,4 +660,75 @@ void single_step_once(struct pt_regs *regs)
 		regs->pc += 8;
 }
 
+#else
+#include <linux/smp.h>
+#include <linux/ptrace.h>
+#include <arch/spr_def.h>
+
+static DEFINE_PER_CPU(unsigned long, ss_saved_pc);
+
+
+/*
+ * Called directly on the occasion of an interrupt.
+ *
+ * If the process doesn't have single step set, then we use this as an
+ * opportunity to turn single step off.
+ *
+ * It has been mentioned that we could conditionally turn off single stepping
+ * on each entry into the kernel and rely on single_step_once to turn it
+ * on for the processes that matter (as we already do), but this
+ * implementation is somewhat more efficient in that we muck with registers
+ * once on a bum interrupt rather than on every entry into the kernel.
+ *
+ * If SINGLE_STEP_CONTROL_K has CANCELED set, then an interrupt occurred,
+ * so we have to run through this process again before we can say that an
+ * instruction has executed.
+ *
+ * swint will set CANCELED, but it's a legitimate instruction.  Fortunately
+ * it changes the PC.  If it hasn't changed, then we know that the interrupt
+ * wasn't generated by swint and we'll need to run this process again before
+ * we can say an instruction has executed.
+ *
+ * If either CANCELED == 0 or the PC's changed, we send out SIGTRAPs and get
+ * on with our lives.
+ */
+
+void gx_singlestep_handle(struct pt_regs *regs, int fault_num)
+{
+	unsigned long *ss_pc = &__get_cpu_var(ss_saved_pc);
+	struct thread_info *info = (void *)current_thread_info();
+	int is_single_step = test_ti_thread_flag(info, TIF_SINGLESTEP);
+	unsigned long control = __insn_mfspr(SPR_SINGLE_STEP_CONTROL_K);
+
+	if (is_single_step == 0) {
+		__insn_mtspr(SPR_SINGLE_STEP_EN_K_K, 0);
+
+	} else if ((*ss_pc != regs->pc) ||
+		   (!(control & SPR_SINGLE_STEP_CONTROL_1__CANCELED_MASK))) {
+
+		ptrace_notify(SIGTRAP);
+		control |= SPR_SINGLE_STEP_CONTROL_1__CANCELED_MASK;
+		control |= SPR_SINGLE_STEP_CONTROL_1__INHIBIT_MASK;
+		__insn_mtspr(SPR_SINGLE_STEP_CONTROL_K, control);
+	}
+}
+
+
+/*
+ * Called from need_singlestep.  Set up the control registers and the enable
+ * register, then return back.
+ */
+
+void single_step_once(struct pt_regs *regs)
+{
+	unsigned long *ss_pc = &__get_cpu_var(ss_saved_pc);
+	unsigned long control = __insn_mfspr(SPR_SINGLE_STEP_CONTROL_K);
+
+	*ss_pc = regs->pc;
+	control |= SPR_SINGLE_STEP_CONTROL_1__CANCELED_MASK;
+	control |= SPR_SINGLE_STEP_CONTROL_1__INHIBIT_MASK;
+	__insn_mtspr(SPR_SINGLE_STEP_CONTROL_K, control);
+	__insn_mtspr(SPR_SINGLE_STEP_EN_K_K, 1 << USER_PL);
+}
+
 #endif /* !__tilegx__ */
diff --git a/arch/tile/kernel/traps.c b/arch/tile/kernel/traps.c
index 7826a8b1..5474fc2 100644
--- a/arch/tile/kernel/traps.c
+++ b/arch/tile/kernel/traps.c
@@ -260,7 +260,7 @@ void __kprobes do_trap(struct pt_regs *regs, int fault_num,
 		address = regs->pc;
 		break;
 	case INT_UNALIGN_DATA:
-#ifndef __tilegx__  /* FIXME: GX: no single-step yet */
+#ifndef __tilegx__  /* Emulated support for single step debugging */
 		if (unaligned_fixup >= 0) {
 			struct single_step_state *state =
 				current_thread_info()->step_state;