tty/serial: at91: add support to FIFOs

Depending on the hardware, TX and RX FIFOs may be available. The RX
FIFO can avoid receive overruns, especially when DMA transfers are
not used to read data from the Receive Holding Register. For heavy
system load, The CPU is likely not be able to fetch data fast enough
from the RHR.

In addition, the RX FIFO can supersede the DMA/PDC to control the RTS
line when the Hardware Handshaking mode is enabled. Two thresholds
are to be set for that purpose:
- When the number of data in the RX FIFO crosses and becomes lower
  than or equal to the low threshold, the RTS line is set to low
  level: the remote peer is requested to send data.
- When the number of data in the RX FIFO crosses and becomes greater
  than or equal to the high threshold, the RTS line is set to high
  level: the remote peer should stop sending new data.
- low threshold <= high threshold
Once these two thresholds are set properly, this new feature is
enabled by setting the FIFO RTS Control bit of the FIFO Mode Register.

FIFOs also introduce a new multiple data mode: the USART works either
in multiple data mode or in single data (legacy) mode.

If MODE9 bit is set into the Mode Register or if USMODE is set to
either LIN_MASTER, LIN_SLAVE or LON_MODE, FIFOs operate in single
data mode. Otherwise, they operate in multiple data mode.

In this new multiple data mode, accesses to the Receive Holding
Register or Transmit Holding Register slightly change.

Since this driver implements neither the 9bit data feature (MODE9 bit
set into the Mode Register) nor LIN modes, the USART works in
multiple data mode whenever FIFOs are available and enabled. We also
assume that data are 8bit wide.

In single data mode, 32bit access CAN be used to read a single data
from RHR or write a single data into THR.
However in multiple data mode, a 32bit access to RHR now allows us to
read four consecutive data from RX FIFO. Also a 32bit access to THR
now allows to write four consecutive data into TX FIFO. So we MUST
use 8bit access whenever only one data have to be read/written at a
time.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 94 insertions, 6 deletions

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index e7c337d..87de21f 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -56,6 +56,15 @@
 /* Revisit: We should calculate this based on the actual port settings */
 #define PDC_RX_TIMEOUT		(3 * 10)		/* 3 bytes */
 
+/* The minium number of data FIFOs should be able to contain */
+#define ATMEL_MIN_FIFO_SIZE	8
+/*
+ * These two offsets are substracted from the RX FIFO size to define the RTS
+ * high and low thresholds
+ */
+#define ATMEL_RTS_HIGH_OFFSET	16
+#define ATMEL_RTS_LOW_OFFSET	20
+
 #if defined(CONFIG_SERIAL_ATMEL_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
 #define SUPPORT_SYSRQ
 #endif
@@ -141,6 +150,9 @@ struct atmel_uart_port {
 	struct mctrl_gpios	*gpios;
 	int			gpio_irq[UART_GPIO_MAX];
 	unsigned int		tx_done_mask;
+	u32			fifo_size;
+	u32			rts_high;
+	u32			rts_low;
 	bool			ms_irq_enabled;
 	bool			is_usart;	/* usart or uart */
 	struct timer_list	uart_timer;	/* uart timer */
@@ -191,6 +203,16 @@ static inline void atmel_uart_writel(struct uart_port *port, u32 reg, u32 value)
 	__raw_writel(value, port->membase + reg);
 }
 
+static inline u8 atmel_uart_readb(struct uart_port *port, u32 reg)
+{
+	return __raw_readb(port->membase + reg);
+}
+
+static inline void atmel_uart_writeb(struct uart_port *port, u32 reg, u8 value)
+{
+	__raw_writeb(value, port->membase + reg);
+}
+
 #ifdef CONFIG_SERIAL_ATMEL_PDC
 static bool atmel_use_pdc_rx(struct uart_port *port)
 {
@@ -635,7 +657,7 @@ static void atmel_rx_chars(struct uart_port *port)
 
 	status = atmel_uart_readl(port, ATMEL_US_CSR);
 	while (status & ATMEL_US_RXRDY) {
-		ch = atmel_uart_readl(port, ATMEL_US_RHR);
+		ch = atmel_uart_readb(port, ATMEL_US_RHR);
 
 		/*
 		 * note that the error handling code is
@@ -686,7 +708,7 @@ static void atmel_tx_chars(struct uart_port *port)
 
 	if (port->x_char &&
 	    (atmel_uart_readl(port, ATMEL_US_CSR) & atmel_port->tx_done_mask)) {
-		atmel_uart_writel(port, ATMEL_US_THR, port->x_char);
+		atmel_uart_writeb(port, ATMEL_US_THR, port->x_char);
 		port->icount.tx++;
 		port->x_char = 0;
 	}
@@ -695,7 +717,7 @@ static void atmel_tx_chars(struct uart_port *port)
 
 	while (atmel_uart_readl(port, ATMEL_US_CSR) &
 	       atmel_port->tx_done_mask) {
-		atmel_uart_writel(port, ATMEL_US_THR, xmit->buf[xmit->tail]);
+		atmel_uart_writeb(port, ATMEL_US_THR, xmit->buf[xmit->tail]);
 		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
 		port->icount.tx++;
 		if (uart_circ_empty(xmit))
@@ -1796,6 +1818,29 @@ static int atmel_startup(struct uart_port *port)
 			atmel_set_ops(port);
 	}
 
+	/*
+	 * Enable FIFO when available
+	 */
+	if (atmel_port->fifo_size) {
+		unsigned int txrdym = ATMEL_US_ONE_DATA;
+		unsigned int rxrdym = ATMEL_US_ONE_DATA;
+		unsigned int fmr;
+
+		atmel_uart_writel(port, ATMEL_US_CR,
+				  ATMEL_US_FIFOEN |
+				  ATMEL_US_RXFCLR |
+				  ATMEL_US_TXFLCLR);
+
+		fmr = ATMEL_US_TXRDYM(txrdym) | ATMEL_US_RXRDYM(rxrdym);
+		if (atmel_port->rts_high &&
+		    atmel_port->rts_low)
+			fmr |=	ATMEL_US_FRTSC |
+				ATMEL_US_RXFTHRES(atmel_port->rts_high) |
+				ATMEL_US_RXFTHRES2(atmel_port->rts_low);
+
+		atmel_uart_writel(port, ATMEL_US_FMR, fmr);
+	}
+
 	/* Save current CSR for comparison in atmel_tasklet_func() */
 	atmel_port->irq_status_prev = atmel_get_lines_status(port);
 	atmel_port->irq_status = atmel_port->irq_status_prev;
@@ -2213,7 +2258,7 @@ static int atmel_poll_get_char(struct uart_port *port)
 	while (!(atmel_uart_readl(port, ATMEL_US_CSR) & ATMEL_US_RXRDY))
 		cpu_relax();
 
-	return atmel_uart_readl(port, ATMEL_US_RHR);
+	return atmel_uart_readb(port, ATMEL_US_RHR);
 }
 
 static void atmel_poll_put_char(struct uart_port *port, unsigned char ch)
@@ -2221,7 +2266,7 @@ static void atmel_poll_put_char(struct uart_port *port, unsigned char ch)
 	while (!(atmel_uart_readl(port, ATMEL_US_CSR) & ATMEL_US_TXRDY))
 		cpu_relax();
 
-	atmel_uart_writel(port, ATMEL_US_THR, ch);
+	atmel_uart_writeb(port, ATMEL_US_THR, ch);
 }
 #endif
 
@@ -2328,7 +2373,7 @@ static void atmel_console_putchar(struct uart_port *port, int ch)
 {
 	while (!(atmel_uart_readl(port, ATMEL_US_CSR) & ATMEL_US_TXRDY))
 		cpu_relax();
-	atmel_uart_writel(port, ATMEL_US_THR, ch);
+	atmel_uart_writeb(port, ATMEL_US_THR, ch);
 }
 
 /*
@@ -2603,6 +2648,48 @@ static int atmel_init_gpios(struct atmel_uart_port *p, struct device *dev)
 	return 0;
 }
 
+static void atmel_serial_probe_fifos(struct atmel_uart_port *port,
+				     struct platform_device *pdev)
+{
+	port->fifo_size = 0;
+	port->rts_low = 0;
+	port->rts_high = 0;
+
+	if (of_property_read_u32(pdev->dev.of_node,
+				 "atmel,fifo-size",
+				 &port->fifo_size))
+		return;
+
+	if (!port->fifo_size)
+		return;
+
+	if (port->fifo_size < ATMEL_MIN_FIFO_SIZE) {
+		port->fifo_size = 0;
+		dev_err(&pdev->dev, "Invalid FIFO size\n");
+		return;
+	}
+
+	/*
+	 * 0 <= rts_low <= rts_high <= fifo_size
+	 * Once their CTS line asserted by the remote peer, some x86 UARTs tend
+	 * to flush their internal TX FIFO, commonly up to 16 data, before
+	 * actually stopping to send new data. So we try to set the RTS High
+	 * Threshold to a reasonably high value respecting this 16 data
+	 * empirical rule when possible.
+	 */
+	port->rts_high = max_t(int, port->fifo_size >> 1,
+			       port->fifo_size - ATMEL_RTS_HIGH_OFFSET);
+	port->rts_low  = max_t(int, port->fifo_size >> 2,
+			       port->fifo_size - ATMEL_RTS_LOW_OFFSET);
+
+	dev_info(&pdev->dev, "Using FIFO (%u data)\n",
+		 port->fifo_size);
+	dev_dbg(&pdev->dev, "RTS High Threshold : %2u data\n",
+		port->rts_high);
+	dev_dbg(&pdev->dev, "RTS Low Threshold  : %2u data\n",
+		port->rts_low);
+}
+
 static int atmel_serial_probe(struct platform_device *pdev)
 {
 	struct atmel_uart_port *port;
@@ -2639,6 +2726,7 @@ static int atmel_serial_probe(struct platform_device *pdev)
 	port = &atmel_ports[ret];
 	port->backup_imr = 0;
 	port->uart.line = ret;
+	atmel_serial_probe_fifos(port, pdev);
 
 	spin_lock_init(&port->lock_suspended);