When only one single character is sent and RS485 signaling is used,
the driver runs into timing issues.
When serial8250_tx_chars is called the single character is transmitted.
The check on uart_circ_empty will be positive and __stop_tx is called.
The check on UART_LSR_TEMT in BOTH_EMPTY will then be negativ and the
function will return. On the next call to serial8250_tx_chars
uart_circ_empty will still be true but the check on BOTH_EMPTY in
__stop_tx might still fail. This leads to a deadlock.
Use readx_poll_timeout_atomic to allow the shift register to be emptied
before checking on BOTH_EMPTY.
The timeout value is copied from 8250_dw.c.
Signed-off-by: Steffen Trumtrar <s.trumtrar@xxxxxxxxxxxxxx>
---
drivers/tty/serial/8250/8250_port.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 3310c2b70138..87daf3758ff0 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -18,6 +18,7 @@
#include <linux/console.h>
#include <linux/gpio/consumer.h>
#include <linux/sysrq.h>
+#include <linux/iopoll.h>
#include <linux/delay.h>
#include <linux/platform_device.h>
#include <linux/tty.h>
@@ -1519,18 +1520,27 @@ static inline void __do_stop_tx(struct uart_8250_port *p)
serial8250_rpm_put_tx(p);
}
+static unsigned char serial8250_read_lsr(struct uart_8250_port *p)
+{
+ return serial_in(p, UART_LSR);
+}
+
static inline void __stop_tx(struct uart_8250_port *p)
{
struct uart_8250_em485 *em485 = p->em485;
if (em485) {
- unsigned char lsr = serial_in(p, UART_LSR);
+ unsigned char 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.
*/
+ readx_poll_timeout_atomic(serial8250_read_lsr, p, lsr,
+ lsr & UART_LSR_TEMT, 1, 20000);
+
if ((lsr & BOTH_EMPTY) != BOTH_EMPTY)
return;
--
2.20.1
