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: raspberrypi#1280

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

Documentation/devicetree/bindings/serial/pl011.txt

@@ -35,6 +35,9 @@ Optional properties:
 - poll-timeout-ms:
 	   Poll timeout when auto-poll is set, default
 	   3000ms.
+- cts-event-workaround:
+	   Enables the (otherwise vendor-specific) workaround for the
+	   CTS-induced TX lockup.
 
 See also bindings/arm/primecell.txt
 

drivers/tty/serial/amba-pl011.c

@@ -2665,6 +2665,11 @@ 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);