[PATCH] Serial: Mobility's 16550A ports need a helping hand

The Mobility 16550A serial ports don't behave the same as standard
16550A ports, and need a helping hand to get them going once the
transmitter has drained and been disabled.

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

2 files changed, 32 insertions, 0 deletions

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 27cc288e91d0..341c644591ae 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -1027,6 +1027,8 @@ static void serial8250_stop_tx(struct uart_port *port, unsigned int tty_stop)
         }
 }
 
+static void transmit_chars(struct uart_8250_port *up);
+
 static void serial8250_start_tx(struct uart_port *port, unsigned int tty_start)
 {
         struct uart_8250_port *up = (struct uart_8250_port *)port;
@@ -1034,6 +1036,14 @@ static void serial8250_start_tx(struct uart_port *port, unsigned int tty_start)
         if (!(up->ier & UART_IER_THRI)) {
                 up->ier |= UART_IER_THRI;
                 serial_out(up, UART_IER, up->ier);
+
+                if (up->capabilities & UART_BUG_TXEN) {
+                        unsigned char lsr, iir;
+                        lsr = serial_in(up, UART_LSR);
+                        iir = serial_in(up, UART_IIR);
+                        if (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT)
+                                transmit_chars(up);
+                }
         }
         /*
          * We only do this from uart_start
@@ -1439,6 +1449,7 @@ static int serial8250_startup(struct uart_port *port)
 {
         struct uart_8250_port *up = (struct uart_8250_port *)port;
         unsigned long flags;
+        unsigned char lsr, iir;
         int retval;
 
         up->capabilities = uart_config[up->port.type].flags;
@@ -1542,6 +1553,26 @@ static int serial8250_startup(struct uart_port *port)
                         up->port.mctrl |= TIOCM_OUT2;
 
         serial8250_set_mctrl(&up->port, up->port.mctrl);
+
+        /*
+         * Do a quick test to see if we receive an
+         * interrupt when we enable the TX irq.
+         */
+        serial_outp(up, UART_IER, UART_IER_THRI);
+        lsr = serial_in(up, UART_LSR);
+        iir = serial_in(up, UART_IIR);
+        serial_outp(up, UART_IER, 0);
+
+        if (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT) {
+                if (!(up->capabilities & UART_BUG_TXEN)) {
+                        up->capabilities |= UART_BUG_TXEN;
+                        pr_debug("ttyS%d - enabling bad tx status workarounds\n",
+                                 port->line);
+                }
+        } else {
+                up->capabilities &= ~UART_BUG_TXEN;
+        }
+
         spin_unlock_irqrestore(&up->port.lock, flags);
 
         /*
diff --git a/drivers/serial/8250.h b/drivers/serial/8250.h
index cd5c3dd2d910..9225c82faeb8 100644
--- a/drivers/serial/8250.h
+++ b/drivers/serial/8250.h
@@ -52,6 +52,7 @@ struct serial8250_config {
 #define UART_CAP_UUE    (1 << 12)       /* UART needs IER bit 6 set (Xscale) */
 
 #define UART_BUG_QUOT   (1 << 0)        /* UART has buggy quot LSB */
+#define UART_BUG_TXEN   (1 << 1)        /* UART has buggy TX IIR status */
 
 #if defined(__i386__) && (defined(CONFIG_M386) || defined(CONFIG_M486))
 #define _INLINE_ inline