serial: 8250_dw: Fix get_mctrl behaviour

Fixed behaviour of get_mctrl() serial driver function as documented in:
https://www.kernel.org/doc/Documentation/serial/driver

Added device-tree properties 'dcd-override', 'dsr-override',
'cts-override', and 'ri-override' specific to the Synopsis 8250
DesignWare UART driver. Allows one to force Data Carrier Detect,
Clear To Send, and Data Set Ready signals to permanently be reported as
active. The Ring indicator can be forced to be reported as inactive.

It is possible that if modem control signalling is enabled on a port
that doesn't have these pins (e.g. - a simple two wire Tx/Rx port), the
driver can hang indefinitely waiting for the state to change. The new
DT properties allow the driver to ignore the state of these pins on
serial ports that don't support them, as recommended in the kernel
documentation.

Reviewed-by: JD (Jiandong) Zheng <jdzheng@broadcom.com>
Signed-off-by: Jonathan Richardson <jonathar@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index e60116235836..2ab229ddee38 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -59,6 +59,8 @@ struct dw8250_data {
 	u8			usr_reg;
 	int			last_mcr;
 	int			line;
+	int			msr_mask_on;
+	int			msr_mask_off;
 	struct clk		*clk;
 	struct clk		*pclk;
 	struct reset_control	*rst;
@@ -81,6 +83,12 @@ static inline int dw8250_modify_msr(struct uart_port *p, int offset, int value)
 		value &= ~UART_MSR_DCTS;
 	}
 
+	/* Override any modem control signals if needed */
+	if (offset == UART_MSR) {
+		value |= d->msr_mask_on;
+		value &= ~d->msr_mask_off;
+	}
+
 	return value;
 }
 
@@ -334,6 +342,30 @@ static int dw8250_probe_of(struct uart_port *p,
 	if (id >= 0)
 		p->line = id;
 
+	if (of_property_read_bool(np, "dcd-override")) {
+		/* Always report DCD as active */
+		data->msr_mask_on |= UART_MSR_DCD;
+		data->msr_mask_off |= UART_MSR_DDCD;
+	}
+
+	if (of_property_read_bool(np, "dsr-override")) {
+		/* Always report DSR as active */
+		data->msr_mask_on |= UART_MSR_DSR;
+		data->msr_mask_off |= UART_MSR_DDSR;
+	}
+
+	if (of_property_read_bool(np, "cts-override")) {
+		/* Always report DSR as active */
+		data->msr_mask_on |= UART_MSR_DSR;
+		data->msr_mask_off |= UART_MSR_DDSR;
+	}
+
+	if (of_property_read_bool(np, "ri-override")) {
+		/* Always report Ring indicator as inactive */
+		data->msr_mask_off |= UART_MSR_RI;
+		data->msr_mask_off |= UART_MSR_TERI;
+	}
+
 	/* clock got configured through clk api, all done */
 	if (p->uartclk)
 		return 0;