TTY: hvc_dcc: probe for a JTAG connection before registering

Enabling the ARM DCC console and using without a JTAG connection will
simply hang the system. Since distros like to turn on all options, this
is a reoccurring problem to debug. We can do better by checking if
anything is attached and handling characters. There is no way to probe
this, so send a newline and check that it is handled.

Cc: Paolo Pisati <paolo.pisati@canonical.com>
Cc: Tim Gardner <tim.gardner@canonical.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/hvc/hvc_dcc.c

@@ -86,6 +86,21 @@ static int hvc_dcc_get_chars(uint32_t vt, char *buf, int count)
 	return i;
 }
 
+static bool hvc_dcc_check(void)
+{
+	unsigned long time = jiffies + (HZ / 10);
+
+	/* Write a test character to check if it is handled */
+	__dcc_putchar('\n');
+
+	while (time_is_after_jiffies(time)) {
+		if (!(__dcc_getstatus() & DCC_STATUS_TX))
+			return true;
+	}
+
+	return false;
+}
+
 static const struct hv_ops hvc_dcc_get_put_ops = {
 	.get_chars = hvc_dcc_get_chars,
 	.put_chars = hvc_dcc_put_chars,
@@ -93,6 +108,9 @@ static const struct hv_ops hvc_dcc_get_put_ops = {
 
 static int __init hvc_dcc_console_init(void)
 {
+	if (!hvc_dcc_check())
+		return -ENODEV;
+
 	hvc_instantiate(0, 0, &hvc_dcc_get_put_ops);
 	return 0;
 }
@@ -100,6 +118,9 @@ console_initcall(hvc_dcc_console_init);
 
 static int __init hvc_dcc_init(void)
 {
+	if (!hvc_dcc_check())
+		return -ENODEV;
+
 	hvc_alloc(0, 0, &hvc_dcc_get_put_ops, 128);
 	return 0;
 }