tty/serial: Fix break handling for PORT_TEGRA

When a break is received, Tegra's UART apparently fills the FIFO with
0 bytes. These must be drained so that they aren't interpreted as actual
data received. This allows e.g. MAGIC_SYSRQ to work on Tegra's UARTs.

v2: Added FIXME comment to clear_rx_fifo

Originally-by: Laxman Dewangan <ldewangan@nvidia.com>
Cc: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 29 insertions, 0 deletions

diff --git a/drivers/tty/serial/8250.c b/drivers/tty/serial/8250.c
index a5e290de8c93..b40f7b90c81d 100644
--- a/drivers/tty/serial/8250.c
+++ b/drivers/tty/serial/8250.c
@@ -1433,6 +1433,27 @@ static void serial8250_enable_ms(struct uart_port *port)
         serial_out(up, UART_IER, up->ier);
 }
 
+/*
+ * Clear the Tegra rx fifo after a break
+ *
+ * FIXME: This needs to become a port specific callback once we have a
+ * framework for this
+ */
+static void clear_rx_fifo(struct uart_8250_port *up)
+{
+        unsigned int status, tmout = 10000;
+        do {
+                status = serial_in(up, UART_LSR);
+                if (status & (UART_LSR_FIFOE | UART_LSR_BRK_ERROR_BITS))
+                        status = serial_in(up, UART_RX);
+                else
+                        break;
+                if (--tmout == 0)
+                        break;
+                udelay(1);
+        } while (1);
+}
+
 static void
 receive_chars(struct uart_8250_port *up, unsigned int *status)
 {
@@ -1468,6 +1489,13 @@ receive_chars(struct uart_8250_port *up, unsigned int *status)
                                 lsr &= ~(UART_LSR_FE | UART_LSR_PE);
                                 up->port.icount.brk++;
                                 /*
+                                 * If tegra port then clear the rx fifo to
+                                 * accept another break/character.
+                                 */
+                                if (up->port.type == PORT_TEGRA)
+                                        clear_rx_fifo(up);
+
+                                /*
                                  * We do the SysRQ and SAK checking
                                  * here because otherwise the break
                                  * may get masked by ignore_status_mask
diff --git a/include/linux/serial_reg.h b/include/linux/serial_reg.h
index 5f66e8499fb9..c75bda37c18e 100644
--- a/include/linux/serial_reg.h
+++ b/include/linux/serial_reg.h
@@ -119,6 +119,7 @@
 #define UART_MCR_DTR            0x01 /* DTR complement */
 
 #define UART_LSR        5       /* In:  Line Status Register */
+#define UART_LSR_FIFOE          0x80 /* Fifo error */
 #define UART_LSR_TEMT           0x40 /* Transmitter empty */
 #define UART_LSR_THRE           0x20 /* Transmit-hold-register empty */
 #define UART_LSR_BI             0x10 /* Break interrupt indicator */