amba_pl011: Add cts-event-workaround DT property

The BCM2835 PL011 implementation seems to have a bug that can lead to a
transmission lockup if CTS changes frequently. A workaround was added to
the driver with a vendor-specific flag to enable it, but this flag is
currently not set for ARM implementations.

Add a "cts-event-workaround" property to Pi DTBs and use the presence
of that property to force the flag to be enabled in the driver.

See: https://github.com/raspberrypi/linux/issues/1280

Signed-off-by: Phil Elwell <phil@raspberrypi.org>

Documentation/devicetree/bindings/serial/pl011.yaml

@@ -104,6 +104,12 @@ properties:
       on the device.
     enum: [1, 4]
 
+  cts-event-workaround:
+    description:
+      Enables the (otherwise vendor-specific) workaround for the
+      CTS-induced TX lockup.
+    type: boolean
+
 required:
   - compatible
   - reg

drivers/tty/serial/amba-pl011.c

@@ -2895,6 +2895,11 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
 	if (IS_ERR(uap->clk))
 		return PTR_ERR(uap->clk);
 
+	if (of_property_read_bool(dev->dev.of_node, "cts-event-workaround")) {
+	    vendor->cts_event_workaround = true;
+	    dev_info(&dev->dev, "cts_event_workaround enabled\n");
+	}
+
 	uap->reg_offset = vendor->reg_offset;
 	uap->vendor = vendor;
 	uap->fifosize = vendor->get_fifosize(dev);