[PATCH v6 2/3] tty: Add software emulated RS485 support for 8250

Matwey V. Kornilov
2015-12-21T18:26:18Z

Implementation of software emulation of RS485 direction handling is based
on omap_serial driver.
Before and after transmission RTS is set to the appropriate value.

Note that before calling serial8250_em485_init the caller has to
ensure that UART will interrupt when shift register empty. Otherwise,
emultaion cannot be used.

Both serial8250_em485_init and serial8250_em485_destroy are
idempotent functions.

Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
---
drivers/tty/serial/8250/8250.h | 6 ++
drivers/tty/serial/8250/8250_port.c | 161 +++++++++++++++++++++++++++++++++++-
include/linux/serial_8250.h | 7 ++
3 files changed, 170 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index d54dcd8..0189cb3 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -117,6 +117,12 @@ static inline void serial_dl_write(struct uart_8250_port *up, int value)
struct uart_8250_port *serial8250_get_port(int line);
void serial8250_rpm_get(struct uart_8250_port *p);
void serial8250_rpm_put(struct uart_8250_port *p);
+int serial8250_em485_init(struct uart_8250_port *p);
+void serial8250_em485_destroy(struct uart_8250_port *p);
+static inline bool serial8250_em485_enabled(struct uart_8250_port *p)
+{
+ return p->em485 && (p->port.rs485.flags & SER_RS485_ENABLED);
+}

#if defined(__alpha__) && !defined(CONFIG_PCI)
/*
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 8ad0b2d..d67a848 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -37,6 +37,7 @@
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/pm_runtime.h>
+#include <linux/timer.h>

#include <asm/io.h>
#include <asm/irq.h>
@@ -504,6 +505,31 @@ static void serial8250_clear_fifos(struct uart_8250_port *p)
}
}

+static inline void serial8250_em485_rts_on_send(struct uart_8250_port *p)
+{
+ unsigned char mcr = serial_in(p, UART_MCR);
+
+ if (p->port.rs485.flags & SER_RS485_RTS_ON_SEND)
+ mcr |= UART_MCR_RTS;
+ else
+ mcr &= ~UART_MCR_RTS;
+ serial_out(p, UART_MCR, mcr);
+}
+
+static inline void serial8250_em485_rts_after_send(struct uart_8250_port *p)
+{
+ unsigned char mcr = serial_in(p, UART_MCR);
+
+ if (p->port.rs485.flags & SER_RS485_RTS_AFTER_SEND)
+ mcr |= UART_MCR_RTS;
+ else
+ mcr &= ~UART_MCR_RTS;
+ serial_out(p, UART_MCR, mcr);
+}
+
+static void serial8250_em485_handle_start_tx(unsigned long arg);
+static void serial8250_em485_handle_stop_tx(unsigned long arg);
+
void serial8250_clear_and_reinit_fifos(struct uart_8250_port *p)
{
serial8250_clear_fifos(p);
@@ -528,6 +554,42 @@ void serial8250_rpm_put(struct uart_8250_port *p)
}
EXPORT_SYMBOL_GPL(serial8250_rpm_put);

+int serial8250_em485_init(struct uart_8250_port *p)
+{
+ if (p->em485 != NULL)
+ return 0;
+
+ p->em485 = kmalloc(sizeof(struct uart_8250_em485), GFP_KERNEL);
+ if (p->em485 == NULL)
+ return -ENOMEM;
+
+ init_timer(&p->em485->stop_tx_timer);
+ p->em485->stop_tx_timer.function = serial8250_em485_handle_stop_tx;
+ p->em485->stop_tx_timer.data = (unsigned long)p;
+ p->em485->stop_tx_timer.flags |= TIMER_IRQSAFE;
+ init_timer(&p->em485->start_tx_timer);
+ p->em485->start_tx_timer.function = serial8250_em485_handle_start_tx;
+ p->em485->start_tx_timer.data = (unsigned long)p;
+ p->em485->start_tx_timer.flags |= TIMER_IRQSAFE;
+
+ serial8250_em485_rts_after_send(p);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(serial8250_em485_init);
+void serial8250_em485_destroy(struct uart_8250_port *p)
+{
+ if (p->em485 == NULL)
+ return;
+
+ del_timer_sync(&p->em485->start_tx_timer);
+ del_timer_sync(&p->em485->stop_tx_timer);
+
+ kfree(p->em485);
+ p->em485 = NULL;
+}
+EXPORT_SYMBOL_GPL(serial8250_em485_destroy);
+
/*
* These two wrappers ensure that enable_runtime_pm_tx() can be called more than
* once and disable_runtime_pm_tx() will still disable RPM because the fifo is
@@ -1293,7 +1355,61 @@ static void serial8250_stop_rx(struct uart_port *port)
serial8250_rpm_put(up);
}

-static inline void __stop_tx(struct uart_8250_port *p)
+static __u32 __start_tx_rs485(struct uart_8250_port *p)
+{
+ if (!serial8250_em485_enabled(p))
+ return 0;
+
+ if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX))
+ serial8250_stop_rx(&p->port);
+
+ del_timer_sync(&p->em485->stop_tx_timer);
+
+ if (!!(p->port.rs485.flags & SER_RS485_RTS_ON_SEND) != !!(serial_in(p, UART_MCR) & UART_MCR_RTS)) {
+ serial8250_em485_rts_on_send(p);
+ return p->port.rs485.delay_rts_before_send;
+ }
+
+ return 0;
+}
+
+static inline void __do_stop_tx_rs485(struct uart_8250_port *p)
+{
+ if (!serial8250_em485_enabled(p))
+ return;
+
+ serial8250_em485_rts_after_send(p);
+ /*
+ * Empty the RX FIFO, we are not interested in anything
+ * received during the half-duplex transmission.
+ */
+ if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX))
+ serial8250_clear_fifos(p);
+}
+
+static void serial8250_em485_handle_stop_tx(unsigned long arg)
+{
+ struct uart_8250_port *p = (struct uart_8250_port *)arg;
+
+ __do_stop_tx_rs485(p);
+}
+
+static inline void __stop_tx_rs485(struct uart_8250_port *p)
+{
+ if (!serial8250_em485_enabled(p))
+ return;
+
+ del_timer_sync(&p->em485->start_tx_timer);
+
+ /* __do_stop_tx_rs485 is going to set RTS according to config AND flush RX FIFO if required */
+ if (p->port.rs485.delay_rts_after_send > 0) {
+ mod_timer(&p->em485->stop_tx_timer, jiffies + p->port.rs485.delay_rts_after_send * HZ / 1000);
+ } else {
+ __do_stop_tx_rs485(p);
+ }
+}
+
+static inline void __do_stop_tx(struct uart_8250_port *p)
{
if (p->ier & UART_IER_THRI) {
p->ier &= ~UART_IER_THRI;
@@ -1302,6 +1418,21 @@ static inline void __stop_tx(struct uart_8250_port *p)
}
}

+static inline void __stop_tx(struct uart_8250_port *p)
+{
+ if (serial8250_em485_enabled(p)) {
+ unsigned char lsr = serial_in(p, UART_LSR);
+ /* To provide required timeing and allow FIFO transfer,
+ * __stop_tx_rs485 must be called only when both FIFO and shift register
+ * are empty. It is for device driver to enable interrupt on TEMT.
+ */
+ if (!((lsr & UART_LSR_TEMT) && (lsr & UART_LSR_THRE)))
+ return;
+ }
+ __do_stop_tx(p);
+ __stop_tx_rs485(p);
+}
+
static void serial8250_stop_tx(struct uart_port *port)
{
struct uart_8250_port *up = up_to_u8250p(port);
@@ -1319,12 +1450,10 @@ static void serial8250_stop_tx(struct uart_port *port)
serial8250_rpm_put(up);
}

-static void serial8250_start_tx(struct uart_port *port)
+static inline void __start_tx(struct uart_port *port)
{
struct uart_8250_port *up = up_to_u8250p(port);

- serial8250_rpm_get_tx(up);
-
if (up->dma && !up->dma->tx_dma(up))
return;

@@ -1350,6 +1479,30 @@ static void serial8250_start_tx(struct uart_port *port)
}
}

+static void serial8250_em485_handle_start_tx(unsigned long arg)
+{
+ struct uart_8250_port *p = (struct uart_8250_port *)arg;
+
+ __start_tx(&p->port);
+}
+
+static void serial8250_start_tx(struct uart_port *port)
+{
+ struct uart_8250_port *up = up_to_u8250p(port);
+ __u32 delay;
+
+ serial8250_rpm_get_tx(up);
+
+ if (up->em485 && timer_pending(&up->em485->start_tx_timer))
+ return;
+
+ if (up->em485 && (delay = __start_tx_rs485(up))) {
+ mod_timer(&up->em485->start_tx_timer, jiffies + delay * HZ / 1000);
+ } else {
+ __start_tx(port);
+ }
+}
+
static void serial8250_throttle(struct uart_port *port)
{
port->throttle(port);
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index faa0e03..71516ec 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -76,6 +76,11 @@ struct uart_8250_ops {
void (*release_irq)(struct uart_8250_port *);
};

+struct uart_8250_em485 {
+ struct timer_list start_tx_timer; /* "rs485 start tx" timer */
+ struct timer_list stop_tx_timer; /* "rs485 stop tx" timer */
+};
+
/*
* This should be used by drivers which want to register
* their own 8250 ports without registering their own
@@ -122,6 +127,8 @@ struct uart_8250_port {
/* 8250 specific callbacks */
int (*dl_read)(struct uart_8250_port *);
void (*dl_write)(struct uart_8250_port *, int);
+
+ struct uart_8250_em485 *em485;
};

static inline struct uart_8250_port *up_to_u8250p(struct uart_port *up)
--
2.6.3