serial: 8250: Set Altera 16550 TX FIFO Threshold

The Altera 16550 soft IP UART requires 2 additional registers for
TX FIFO threshold support. These 2 registers enable the TX FIFO
Low Watermark and set the TX FIFO Low Watermark.
Set the TX FIFO threshold to the FIFO size - tx_loadsz.

Signed-off-by: Thor Thayer <tthayer@opensource.altera.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 43 insertions, 0 deletions

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 28d29fac1973..1bfb6fdbaa20 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1870,6 +1870,30 @@ static int exar_handle_irq(struct uart_port *port)
         return ret;
 }
 
+/*
+ * Newer 16550 compatible parts such as the SC16C650 & Altera 16550 Soft IP
+ * have a programmable TX threshold that triggers the THRE interrupt in
+ * the IIR register. In this case, the THRE interrupt indicates the FIFO
+ * has space available. Load it up with tx_loadsz bytes.
+ */
+static int serial8250_tx_threshold_handle_irq(struct uart_port *port)
+{
+        unsigned long flags;
+        unsigned int iir = serial_port_in(port, UART_IIR);
+
+        /* TX Threshold IRQ triggered so load up FIFO */
+        if ((iir & UART_IIR_ID) == UART_IIR_THRI) {
+                struct uart_8250_port *up = up_to_u8250p(port);
+
+                spin_lock_irqsave(&port->lock, flags);
+                serial8250_tx_chars(up);
+                spin_unlock_irqrestore(&port->lock, flags);
+        }
+
+        iir = serial_port_in(port, UART_IIR);
+        return serial8250_handle_irq(port, iir);
+}
+
 static unsigned int serial8250_tx_empty(struct uart_port *port)
 {
         struct uart_8250_port *up = up_to_u8250p(port);
@@ -2159,6 +2183,25 @@ int serial8250_do_startup(struct uart_port *port)
                 serial_port_out(port, UART_LCR, 0);
         }
 
+        /*
+         * For the Altera 16550 variants, set TX threshold trigger level.
+         */
+        if (((port->type == PORT_ALTR_16550_F32) ||
+             (port->type == PORT_ALTR_16550_F64) ||
+             (port->type == PORT_ALTR_16550_F128)) && (port->fifosize > 1)) {
+                /* Bounds checking of TX threshold (valid 0 to fifosize-2) */
+                if ((up->tx_loadsz < 2) || (up->tx_loadsz > port->fifosize)) {
+                        pr_err("ttyS%d TX FIFO Threshold errors, skipping\n",
+                               serial_index(port));
+                } else {
+                        serial_port_out(port, UART_ALTR_AFR,
+                                        UART_ALTR_EN_TXFIFO_LW);
+                        serial_port_out(port, UART_ALTR_TX_LOW,
+                                        port->fifosize - up->tx_loadsz);
+                        port->handle_irq = serial8250_tx_threshold_handle_irq;
+                }
+        }
+
         if (port->irq) {
                 unsigned char iir1;
                 /*