[Serial] Don't miss modem status changes

Reading the MSR register on 8250-compatible UARTs results in any
modem status interrupts being cleared.  To avoid missing any
status changes, arrange for get_mctrl() to read the current
status via check_modem_status(), which will process any pending
state changes for us.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 16 insertions, 17 deletions

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index d2bcd1f87cd6..076bf848e4d1 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -1255,25 +1255,24 @@ static _INLINE_ void transmit_chars(struct uart_8250_port *up)
                 __stop_tx(up);
 }
 
-static _INLINE_ void check_modem_status(struct uart_8250_port *up)
+static unsigned int check_modem_status(struct uart_8250_port *up)
 {
-        int status;
+        unsigned int status = serial_in(up, UART_MSR);
 
-        status = serial_in(up, UART_MSR);
+        if (status & UART_MSR_ANY_DELTA && up->ier & UART_IER_MSI) {
+                if (status & UART_MSR_TERI)
+                        up->port.icount.rng++;
+                if (status & UART_MSR_DDSR)
+                        up->port.icount.dsr++;
+                if (status & UART_MSR_DDCD)
+                        uart_handle_dcd_change(&up->port, status & UART_MSR_DCD);
+                if (status & UART_MSR_DCTS)
+                        uart_handle_cts_change(&up->port, status & UART_MSR_CTS);
 
-        if ((status & UART_MSR_ANY_DELTA) == 0)
-                return;
-
-        if (status & UART_MSR_TERI)
-                up->port.icount.rng++;
-        if (status & UART_MSR_DDSR)
-                up->port.icount.dsr++;
-        if (status & UART_MSR_DDCD)
-                uart_handle_dcd_change(&up->port, status & UART_MSR_DCD);
-        if (status & UART_MSR_DCTS)
-                uart_handle_cts_change(&up->port, status & UART_MSR_CTS);
+                wake_up_interruptible(&up->port.info->delta_msr_wait);
+        }
 
-        wake_up_interruptible(&up->port.info->delta_msr_wait);
+        return status;
 }
 
 /*
@@ -1454,10 +1453,10 @@ static unsigned int serial8250_tx_empty(struct uart_port *port)
 static unsigned int serial8250_get_mctrl(struct uart_port *port)
 {
         struct uart_8250_port *up = (struct uart_8250_port *)port;
-        unsigned char status;
+        unsigned int status;
         unsigned int ret;
 
-        status = serial_in(up, UART_MSR);
+        status = check_modem_status(up);
 
         ret = 0;
         if (status & UART_MSR_DCD)