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 bug by adding a memory barrier after the functions
that access the ISA ports - outb, outw, outl. The barrier causes that
there is some delay between the write to an IO port and a subsequent read.

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+

---
 arch/alpha/include/asm/io.h |    3 +++
 arch/alpha/kernel/io.c      |    3 +++
 2 files changed, 6 insertions(+)

Index: linux-stable/arch/alpha/include/asm/io.h
===================================================================

--- linux-stable.orig/arch/alpha/include/asm/io.h	2020-05-07 17:36:58.000000000 +0200
+++ linux-stable/arch/alpha/include/asm/io.h	2020-05-07 17:36:58.000000000 +0200

@@ -347,11 +347,13 @@ extern inline u16 inw(unsigned long port
 extern inline void outb(u8 b, unsigned long port)
 {
 	iowrite8(b, ioport_map(port, 1));
+	mb();
 }
 
 extern inline void outw(u16 b, unsigned long port)
 {
 	iowrite16(b, ioport_map(port, 2));
+	mb();
 }
 #endif
 

@@ -377,6 +379,7 @@ extern inline u32 inl(unsigned long port
 extern inline void outl(u32 b, unsigned long port)
 {
 	iowrite32(b, ioport_map(port, 4));
+	mb();
 }
 #endif
 

Index: linux-stable/arch/alpha/kernel/io.c
===================================================================

--- linux-stable.orig/arch/alpha/kernel/io.c	2020-05-07 17:36:58.000000000 +0200
+++ linux-stable/arch/alpha/kernel/io.c	2020-05-07 17:36:58.000000000 +0200

@@ -78,16 +78,19 @@ u32 inl(unsigned long port)
 void outb(u8 b, unsigned long port)
 {
 	iowrite8(b, ioport_map(port, 1));
+	mb();
 }
 
 void outw(u16 b, unsigned long port)
 {
 	iowrite16(b, ioport_map(port, 2));
+	mb();
 }
 
 void outl(u32 b, unsigned long port)
 {
 	iowrite32(b, ioport_map(port, 4));
+	mb();
 }
 
 EXPORT_SYMBOL(inb);