thunderbolt: Rework NFC credits handling

NFC (non flow control) credits is actually 20-bit field so update
tb_port_add_nfc_credits() to handle this properly. This allows us to set
NFC credits for Display Port path in subsequent patches.

Also make sure the function does not update the hardware if the
underlying switch is already unplugged.
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/thunderbolt/switch.c

@@ -537,14 +537,20 @@ int tb_wait_for_port(struct tb_port *port, bool wait_if_unplugged)
  */
 int tb_port_add_nfc_credits(struct tb_port *port, int credits)
 {
-	if (credits == 0)
+	u32 nfc_credits;
+
+	if (credits == 0 || port->sw->is_unplugged)
 		return 0;
-	tb_port_info(port,
-		     "adding %#x NFC credits (%#x -> %#x)",
-		     credits,
-		     port->config.nfc_credits,
-		     port->config.nfc_credits + credits);
-	port->config.nfc_credits += credits;
+
+	nfc_credits = port->config.nfc_credits & TB_PORT_NFC_CREDITS_MASK;
+	nfc_credits += credits;
+
+	tb_port_dbg(port, "adding %d NFC credits to %lu",
+		    credits, port->config.nfc_credits & TB_PORT_NFC_CREDITS_MASK);
+
+	port->config.nfc_credits &= ~TB_PORT_NFC_CREDITS_MASK;
+	port->config.nfc_credits |= nfc_credits;
+
 	return tb_port_write(port, &port->config.nfc_credits,
 			     TB_CFG_PORT, 4, 1);
 }

drivers/thunderbolt/tb_regs.h

@@ -211,6 +211,9 @@ struct tb_regs_port_header {
 
 } __packed;
 
+/* DWORD 4 */
+#define TB_PORT_NFC_CREDITS_MASK	GENMASK(19, 0)
+
 /* PCIe adapter registers */
 
 #define TB_PCI_EN			BIT(31)