From: Thor Thayer <tthayer@xxxxxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxxxxx>
---
v2 Add bounds checking of tx threshold value.
---
drivers/tty/serial/8250/8250_port.c | 43 +++++++++++++++++++++++++++++++++++
include/uapi/linux/serial_reg.h | 8 +++++++
2 files changed, 51 insertions(+)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index ae3a104..24ef65a 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);
@@ -2158,6 +2182,25 @@ 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;
/*
diff --git a/include/uapi/linux/serial_reg.h b/include/uapi/linux/serial_reg.h
index 1e5ac4e7..b4c0484 100644
--- a/include/uapi/linux/serial_reg.h
+++ b/include/uapi/linux/serial_reg.h
@@ -376,5 +376,13 @@
#define UART_EXAR_TXTRG 0x0a /* Tx FIFO trigger level write-only */
#define UART_EXAR_RXTRG 0x0b /* Rx FIFO trigger level write-only */
+/*
+ * These are definitions for the Altera ALTR_16550_F32/F64/F128
+ * Normalized from 0x100 to 0x40 because of shift by 2 (32 bit regs).
+ */
+#define UART_ALTR_AFR 0x40 /* Additional Features Register */
+#define UART_ALTR_EN_TXFIFO_LW 0x01 /* Enable the TX FIFO Low Watermark */
+#define UART_ALTR_TX_LOW 0x41 /* Tx FIFO Low Watermark */
+
#endif /* _LINUX_SERIAL_REG_H */
--
1.7.9.5
--
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