From 4fdb86b3236ee368453af71a0fbd392c4276838c Mon Sep 17 00:00:00 2001
From: Robert Nelson <robertcnelson@gmail.com>
Date: Thu, 20 May 2021 16:51:43 -0500
Subject: [PATCH] Revert "Revert "serial: 8250: Fix clearing FIFOs in RS485
mode again""
This reverts commit 3c9dc275dba1124c1e16e7037226038451286813.
Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
---
drivers/tty/serial/8250/8250_port.c | 29 ++++++++++++++++++++++++-----
1 file changed, 24 insertions(+), 5 deletions(-)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 7c07ebb37b1b9..77642b556b39d 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -555,11 +555,30 @@ static unsigned int serial_icr_read(struct uart_8250_port *up, int offset)
*/
static void serial8250_clear_fifos(struct uart_8250_port *p)
{
+ unsigned char fcr;
+ unsigned char clr_mask = UART_FCR_CLEAR_RCVR | UART_FCR_CLEAR_XMIT;
+
if (p->capabilities & UART_CAP_FIFO) {
- serial_out(p, UART_FCR, UART_FCR_ENABLE_FIFO);
- serial_out(p, UART_FCR, UART_FCR_ENABLE_FIFO |
- UART_FCR_CLEAR_RCVR | UART_FCR_CLEAR_XMIT);
- serial_out(p, UART_FCR, 0);
+ /*
+ * Make sure to avoid changing FCR[7:3] and ENABLE_FIFO bits.
+ * In case ENABLE_FIFO is not set, there is nothing to flush
+ * so just return. Furthermore, on certain implementations of
+ * the 8250 core, the FCR[7:3] bits may only be changed under
+ * specific conditions and changing them if those conditions
+ * are not met can have nasty side effects. One such core is
+ * the 8250-omap present in TI AM335x.
+ */
+ fcr = serial_in(p, UART_FCR);
+
+ /* FIFO is not enabled, there's nothing to clear. */
+ if (!(fcr & UART_FCR_ENABLE_FIFO))
+ return;
+
+ fcr |= clr_mask;
+ serial_out(p, UART_FCR, fcr);
+
+ fcr &= ~clr_mask;
+ serial_out(p, UART_FCR, fcr);
}
}
@@ -1463,7 +1482,7 @@ void serial8250_em485_stop_tx(struct uart_8250_port *p)
* Enable previously disabled RX interrupts.
*/
if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX)) {
- serial8250_clear_and_reinit_fifos(p);
+ serial8250_clear_fifos(p);
p->ier |= UART_IER_RLSI | UART_IER_RDI;
serial_port_out(&p->port, UART_IER, p->ier);
--
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