[PATCH v3 3/4] tty/serial: at91: add support to FIFOs

Cyrille Pitchen
2015-06-30T14:06:28Z

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>
---
drivers/tty/serial/atmel_serial.c | 92 ++++++++++++++++++++++++++++++++++++++-
include/linux/atmel_serial.h | 36 +++++++++++++++
2 files changed, 126 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 112e74b61f1f..e3082d912a52 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 8
+#define ATMEL_RTS_LOW_OFFSET 16
+
#if defined(CONFIG_SERIAL_ATMEL_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
#define SUPPORT_SYSRQ
#endif
@@ -96,8 +105,8 @@ static void atmel_stop_rx(struct uart_port *port);
#define UART_PUT_IDR(port, v) __raw_writel(v, (port)->membase + ATMEL_US_IDR)
#define UART_GET_IMR(port) __raw_readl((port)->membase + ATMEL_US_IMR)
#define UART_GET_CSR(port) __raw_readl((port)->membase + ATMEL_US_CSR)
-#define UART_GET_CHAR(port) __raw_readl((port)->membase + ATMEL_US_RHR)
-#define UART_PUT_CHAR(port, v) __raw_writel(v, (port)->membase + ATMEL_US_THR)
+#define UART_GET_CHAR(port) __raw_readb((port)->membase + ATMEL_US_RHR)
+#define UART_PUT_CHAR(port, v) __raw_writeb(v, (port)->membase + ATMEL_US_THR)
#define UART_GET_BRGR(port) __raw_readl((port)->membase + ATMEL_US_BRGR)
#define UART_PUT_BRGR(port, v) __raw_writel(v, (port)->membase + ATMEL_US_BRGR)
#define UART_PUT_RTOR(port, v) __raw_writel(v, (port)->membase + ATMEL_US_RTOR)
@@ -119,6 +128,16 @@ static void atmel_stop_rx(struct uart_port *port);
#define UART_PUT_TCR(port, v) __raw_writel(v, (port)->membase + ATMEL_PDC_TCR)
#define UART_GET_TCR(port) __raw_readl((port)->membase + ATMEL_PDC_TCR)

+/* FIFO registers */
+#define UART_PUT_FMR(port, v) __raw_writel(v, (port)->membase + ATMEL_US_FMR)
+#define UART_GET_FMR(port) __raw_readl((port)->membase + ATMEL_US_FMR)
+#define UART_GET_FESR(port) __raw_readl((port)->membase + ATMEL_US_FESR)
+#define UART_PUT_FIER(port, v) __raw_writel(v, (port)->membase + ATMEL_US_FIER)
+#define UART_PUT_FIDR(port, v) __raw_writel(v, (port)->membase + ATMEL_US_FIDR)
+#define UART_GET_FIMR(port) __raw_readl((port)->membase + ATMEL_US_FIMR)
+#define UART_GET_FLR(port) __raw_readl((port)->membase + ATMEL_US_FLR)
+
+
struct atmel_dma_buffer {
unsigned char *buf;
dma_addr_t dma_addr;
@@ -172,6 +191,9 @@ 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 */
@@ -1797,6 +1819,29 @@ 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;
+
+ UART_PUT_CR(port,
+ 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);
+
+ UART_PUT_FMR(port, 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;
@@ -2599,6 +2644,48 @@ 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 8 data, before
+ * actually stopping to send new data. So we try to set the RTS High
+ * Threshold to a reasonably high value respecting this 8 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_info(&pdev->dev, "RTS High Threshold : %2u data\n",
+ port->rts_high);
+ dev_info(&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;
@@ -2635,6 +2722,7 @@ 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);

diff --git a/include/linux/atmel_serial.h b/include/linux/atmel_serial.h
index c384c21d65f0..ee696d7e8a43 100644
--- a/include/linux/atmel_serial.h
+++ b/include/linux/atmel_serial.h
@@ -35,6 +35,11 @@
#define ATMEL_US_DTRDIS BIT(17) /* Data Terminal Ready Disable */
#define ATMEL_US_RTSEN BIT(18) /* Request To Send Enable */
#define ATMEL_US_RTSDIS BIT(19) /* Request To Send Disable */
+#define ATMEL_US_TXFCLR BIT(24) /* Transmit FIFO Clear */
+#define ATMEL_US_RXFCLR BIT(25) /* Receive FIFO Clear */
+#define ATMEL_US_TXFLCLR BIT(26) /* Transmit FIFO Lock Clear */
+#define ATMEL_US_FIFOEN BIT(30) /* FIFO enable */
+#define ATMEL_US_FIFODIS BIT(31) /* FIFO disable */

#define ATMEL_US_MR 0x04 /* Mode Register */
#define ATMEL_US_USMODE GENMASK(3, 0) /* Mode of the USART */
@@ -124,6 +129,37 @@
#define ATMEL_US_NER 0x44 /* Number of Errors Register */
#define ATMEL_US_IF 0x4c /* IrDA Filter Register */

+#define ATMEL_US_CMPR 0x90 /* Comparaison Register */
+#define ATMEL_US_FMR 0xa0 /* FIFO Mode Register */
+#define ATMEL_US_TXRDYM(data) (((data) & 0x3) << 0) /* TX Ready Mode */
+#define ATMEL_US_RXRDYM(data) (((data) & 0x3) << 4) /* RX Ready Mode */
+#define ATMEL_US_ONE_DATA 0x0
+#define ATMEL_US_TWO_DATA 0x1
+#define ATMEL_US_FOUR_DATA 0x2
+#define ATMEL_US_FRTSC BIT(7) /* FIFO RTS pin Control */
+#define ATMEL_US_TXFTHRES(thr) (((thr) & 0x3f) << 8) /* TX FIFO Threshold */
+#define ATMEL_US_RXFTHRES(thr) (((thr) & 0x3f) << 16) /* RX FIFO Threshold */
+#define ATMEL_US_RXFTHRES2(thr) (((thr) & 0x3f) << 24) /* RX FIFO Threshold2 */
+
+#define ATMEL_US_FLR 0xa4 /* FIFO Level Register */
+#define ATMEL_US_TXFL(reg) (((reg) >> 0) & 0x3f) /* TX FIFO Level */
+#define ATMEL_US_RXFL(reg) (((reg) >> 16) & 0x3f) /* RX FIFO Level */
+
+#define ATMEL_US_FIER 0xa8 /* FIFO Interrupt Enable Register */
+#define ATMEL_US_FIDR 0xac /* FIFO Interrupt Disable Register */
+#define ATMEL_US_FIMR 0xb0 /* FIFO Interrupt Mask Register */
+#define ATMEL_US_FESR 0xb4 /* FIFO Event Status Register */
+#define ATMEL_US_TXFEF BIT(0) /* Transmit FIFO Empty Flag */
+#define ATMEL_US_TXFFF BIT(1) /* Transmit FIFO Full Flag */
+#define ATMEL_US_TXFTHF BIT(2) /* Transmit FIFO Threshold Flag */
+#define ATMEL_US_RXFEF BIT(3) /* Receive FIFO Empty Flag */
+#define ATMEL_US_RXFFF BIT(4) /* Receive FIFO Full Flag */
+#define ATMEL_US_RXFTHF BIT(5) /* Receive FIFO Threshold Flag */
+#define ATMEL_US_TXFPTEF BIT(6) /* Transmit FIFO Pointer Error Flag */
+#define ATMEL_US_RXFPTEF BIT(7) /* Receive FIFO Pointer Error Flag */
+#define ATMEL_US_TXFLOCK BIT(8) /* Transmit FIFO Lock (FESR only) */
+#define ATMEL_US_RXFTHF2 BIT(9) /* Receive FIFO Threshold Flag 2 */
+
#define ATMEL_US_NAME 0xf0 /* Ip Name */
#define ATMEL_US_VERSION 0xfc /* Ip Version */

--
1.8.2.2