[PATCH] serial: meson: handle RX errors

[Neil Armstrong]

This checks and handles RX errors on the Amlogic UART controller
after experiencing errors on the Khadas VIM3 & VIM3L when UART AO A
lines are not connected.

When the RX line is not connected, the first byte is erroneous and breaks
the U-Boot autoboot, breaking automatic boot.

This checks and drops any erroneous RX byte on pending and getc callbacks
to avoid returning true to pending when an error byte is in the FIFO.

Fixes: bfcef28ae4 ("arm: add initial support for Amlogic Meson and ODROID-C2")
Signed-off-by: Neil Armstrong <narmstrong at baylibre.com>
---
 drivers/serial/serial_meson.c | 43 +++++++++++++++++++++++++++++++----
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/drivers/serial/serial_meson.c b/drivers/serial/serial_meson.c
index 439057b1b9..1b1725f6aa 100644
--- a/drivers/serial/serial_meson.c
+++ b/drivers/serial/serial_meson.c
@@ -65,14 +65,36 @@ static int meson_serial_probe(struct udevice *dev)
 	return 0;
 }
 
+static void meson_serial_rx_error(struct udevice *dev)
+{
+	struct meson_serial_platdata *plat = dev->platdata;
+	struct meson_uart *const uart = plat->reg;
+	u32 val = readl(&uart->control);
+
+	/* Clear error */
+	val |= AML_UART_CLR_ERR;
+	writel(val, &uart->control);
+	val &= ~AML_UART_CLR_ERR;
+	writel(val, &uart->control);
+
+	/* Remove spurious byte from fifo */
+	readl(&uart->rfifo);
+}
+
 static int meson_serial_getc(struct udevice *dev)
 {
 	struct meson_serial_platdata *plat = dev->platdata;
 	struct meson_uart *const uart = plat->reg;
+	uint32_t status = readl(&uart->status);
 
-	if (readl(&uart->status) & AML_UART_RX_EMPTY)
+	if (status & AML_UART_RX_EMPTY)
 		return -EAGAIN;
 
+	if (status & AML_UART_ERR) {
+		meson_serial_rx_error(dev);
+		return -EIO;
+	}
+
 	return readl(&uart->rfifo) & 0xff;
 }
 
@@ -95,10 +117,23 @@ static int meson_serial_pending(struct udevice *dev, bool input)
 	struct meson_uart *const uart = plat->reg;
 	uint32_t status = readl(&uart->status);
 
-	if (input)
-		return !(status & AML_UART_RX_EMPTY);
-	else
+	if (input) {
+		if (status & AML_UART_RX_EMPTY)
+			return false;
+
+		/*
+		 * Handle and drop any RX error here to avoid
+		 * returning true here when an error byte is in the FIFO
+		 */
+		if (status & AML_UART_ERR) {
+			meson_serial_rx_error(dev);
+			return false;
+		}
+
+		return true;
+	} else {
 		return !(status & AML_UART_TX_FULL);
+	}
 }
 
 static int meson_serial_ofdata_to_platdata(struct udevice *dev)
-- 
2.22.0