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: #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
 

arch/arm/boot/dts/bcm270x.dtsi

@@ -26,6 +26,11 @@
  interrupts = <2 17>, <2 18>;
  };
 
+ serial@7e201000 { /* uart0 */
+ /* Enable CTS bug workaround */
+ cts-event-workaround;
+ };
+
  i2s@7e203000 { /* i2s */
  #sound-dai-cells = <0>;
  reg = <0x7e203000 0x24>;

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);