[V3,6/6] usb: serial: f81534: fix tx error on some baud rate

[V3,6/6] usb: serial: f81534: fix tx error on some baud rate

[Ji-Ze Hong (Peter Hong)]

The F81532/534 had 4 clocksource 1.846/18.46/14.77/24MHz and baud rates
can be up to 1.5Mbits with 24MHz. But on some baud rate (384~500kps), the
TX side will send the data frame too close to treat frame error on RX
side. This patch will force all TX data frame with delay 1bit gap.

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
---
V3:
	1: had not noticeable changes.

V2:
	1: First introduced in this series patches.

 drivers/usb/serial/f81534.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/serial/f81534.c b/drivers/usb/serial/f81534.c
index 08ef72ea2eec..92c9d8f18a5a 100644
--- a/drivers/usb/serial/f81534.c
+++ b/drivers/usb/serial/f81534.c
@@ -131,6 +131,8 @@
 #define F81534_CLK_24_MHZ		BIT(2)
 #define F81534_CLK_14_77_MHZ		GENMASK(2, 1)
 #define F81534_CLK_MASK			GENMASK(2, 1)
+#define F81534_CLK_TX_DELAY_1BIT	BIT(3)
+
 #define F81534_CLK_RS485_MODE		BIT(4)
 #define F81534_CLK_RS485_INVERT		BIT(5)
 
@@ -1386,7 +1388,11 @@ static int f81534_port_probe(struct usb_serial_port *port)
 	if (!port_priv)
 		return -ENOMEM;
 
-	port_priv->shadow_clk = F81534_UART_EN;
+	/*
+	 * We'll make tx frame error when baud rate from 384~500kps. So we'll
+	 * delay all tx data frame with 1bit.
+	 */
+	port_priv->shadow_clk = F81534_UART_EN | F81534_CLK_TX_DELAY_1BIT;
 	spin_lock_init(&port_priv->msr_lock);
 	mutex_init(&port_priv->mcr_mutex);
 	mutex_init(&port_priv->lcr_mutex);