[PATCH 2/2] alpha: add a delay before serial port read

[Mikulas Patocka <mpatocka@redhat.com>]

The patch 92d7223a74235054f2aa7227d207d9c57f84dca0 ("alpha: io: reorder
barriers to guarantee writeX() and iowriteX() ordering #2") broke boot on
the Alpha Avanti platform.

The patch changes timing between accesses to the ISA bus, in particular,
it reduces the time between "write" access and a subsequent "read" access.

This causes lock-up when accessing the real time clock and serial ports.

This patch fixes the serial ports by adding a small delay before the "inb"
instruction.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 92d7223a7423 ("alpha: io: reorder barriers to guarantee writeX() and iowriteX() ordering #2")
Cc: stable@vger.kernel.org	# v4.17+

---
 drivers/tty/serial/8250/8250_port.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux-stable/drivers/tty/serial/8250/8250_port.c
===================================================================
--- linux-stable.orig/drivers/tty/serial/8250/8250_port.c	2020-05-06 08:25:19.000000000 +0200
+++ linux-stable/drivers/tty/serial/8250/8250_port.c	2020-05-06 09:04:17.000000000 +0200
@@ -442,6 +442,10 @@ static unsigned int mem32be_serial_in(st
 
 static unsigned int io_serial_in(struct uart_port *p, int offset)
 {
+#ifdef CONFIG_ALPHA
+/* we need a small delay, the Alpha Avanti chipset locks up with back-to-back accesses */
+	ndelay(300);
+#endif
 	offset = offset << p->regshift;
 	return inb(p->iobase + offset);
 }