target/riscv: Add periodic tick() callback

Intended as a place where we can interact with the target without too
much concern about preserving state and doing exactly the right thing
while poll() is going on.

Change-Id: Ic9bd441caae85901a131fd45e742599803df89b5
Signed-off-by: Tim Newsome <tim@sifive.com>

src/target/riscv/riscv.c

@@ -2934,6 +2934,17 @@ int riscv_openocd_poll(struct target *target)
 		}
 	}
 
+	/* Call tick() for every hart. What happens in tick() is opaque to this
+	 * layer. The reason it's outside the previous loop is that at this point
+	 * the state of every hart has settled, so any side effects happening in
+	 * tick() won't affect the delicate poll() code. */
+	foreach_smp_target(entry, targets) {
+		struct target *t = entry->target;
+		struct riscv_info *info = riscv_info(t);
+		if (info->tick && info->tick(t) != ERROR_OK)
+			return ERROR_FAIL;
+	}
+
 	/* Sample memory if any target is running. */
 	foreach_smp_target(entry, targets) {
 		struct target *t = entry->target;

src/target/riscv/riscv.h

@@ -204,6 +204,10 @@ struct riscv_info {
 	int (*handle_became_unavailable)(struct target *target,
 		enum riscv_hart_state previous_riscv_state);
 
+	/* Called periodically (no guarantees about frequency), while there's
+	 * nothing else going on. */
+	int (*tick)(struct target *target);
+
 	/* Get this target as ready as possible to resume, without actually
 	 * resuming. */
 	int (*resume_prep)(struct target *target);