[PATCH 2/2 v5] 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.

We introduce a global variable
alpha_has_broken_serial_ports_and_needs_delay - it is defined on Alpha and
it is set to 1 if we have the specific PCI-ISA bridge where this bug
occurs. We also introduce a new per-port flag UPQ_DELAY_BEFORE_READ, the
flag is set if alpha_has_broken_serial_ports_and_needs_delay is set and if
the serial port is an ISA port.

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/serial.h     |    5 +++++
 arch/alpha/kernel/pci.c             |    5 +++++
 drivers/tty/serial/8250/8250_core.c |   17 +++++++++++------
 drivers/tty/serial/8250/8250_port.c |    3 +++
 include/linux/serial_core.h         |    1 +
 5 files changed, 25 insertions(+), 6 deletions(-)

Index: linux-stable/arch/alpha/kernel/pci.c
===================================================================
--- linux-stable.orig/arch/alpha/kernel/pci.c	2020-05-07 09:57:02.000000000 +0200
+++ linux-stable/arch/alpha/kernel/pci.c	2020-05-07 14:30:07.000000000 +0200
@@ -24,6 +24,7 @@
 #include <linux/slab.h>
 #include <linux/syscalls.h>
 #include <asm/machvec.h>
+#include <asm/serial.h>
 
 #include "proto.h"
 #include "pci_impl.h"
@@ -61,9 +62,13 @@ struct pci_controller *pci_isa_hose;
  * Quirks.
  */
 
+int alpha_has_broken_serial_ports_and_needs_delay = 0;
+EXPORT_SYMBOL(alpha_has_broken_serial_ports_and_needs_delay);
+
 static void quirk_isa_bridge(struct pci_dev *dev)
 {
 	dev->class = PCI_CLASS_BRIDGE_ISA << 8;
+	alpha_has_broken_serial_ports_and_needs_delay = 1;
 }
 DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_82378, quirk_isa_bridge);
 
Index: linux-stable/drivers/tty/serial/8250/8250_port.c
===================================================================
--- linux-stable.orig/drivers/tty/serial/8250/8250_port.c	2020-05-07 09:57:02.000000000 +0200
+++ linux-stable/drivers/tty/serial/8250/8250_port.c	2020-05-07 09:57:02.000000000 +0200
@@ -442,6 +442,9 @@ static unsigned int mem32be_serial_in(st
 
 static unsigned int io_serial_in(struct uart_port *p, int offset)
 {
+	if (unlikely(p->quirks & UPQ_DELAY_BEFORE_READ))
+		ndelay(300);
+
 	offset = offset << p->regshift;
 	return inb(p->iobase + offset);
 }
Index: linux-stable/drivers/tty/serial/8250/8250_core.c
===================================================================
--- linux-stable.orig/drivers/tty/serial/8250/8250_core.c	2020-05-07 09:57:02.000000000 +0200
+++ linux-stable/drivers/tty/serial/8250/8250_core.c	2020-05-07 14:34:16.000000000 +0200
@@ -487,9 +487,20 @@ static void univ8250_rsa_support(struct
 #define univ8250_rsa_support(x)		do { } while (0)
 #endif /* CONFIG_SERIAL_8250_RSA */
 
+/*
+ * This "device" covers _all_ ISA 8250-compatible serial devices listed
+ * in the table in include/asm/serial.h
+ */
+static struct platform_device *serial8250_isa_devs;
+
 static inline void serial8250_apply_quirks(struct uart_8250_port *up)
 {
 	up->port.quirks |= skip_txen_test ? UPQ_NO_TXEN_TEST : 0;
+#ifdef alpha_has_broken_serial_ports_and_needs_delay
+	if (alpha_has_broken_serial_ports_and_needs_delay &&
+	    serial8250_isa_devs && up->port.dev == &serial8250_isa_devs->dev)
+		up->port.quirks |= UPQ_DELAY_BEFORE_READ;
+#endif
 }
 
 static void __init serial8250_isa_init_ports(void)
@@ -903,12 +914,6 @@ static struct platform_driver serial8250
 };
 
 /*
- * This "device" covers _all_ ISA 8250-compatible serial devices listed
- * in the table in include/asm/serial.h
- */
-static struct platform_device *serial8250_isa_devs;
-
-/*
  * serial8250_register_8250_port and serial8250_unregister_port allows for
  * 16x50 serial ports to be configured at run-time, to support PCMCIA
  * modems and PCI multiport cards.
Index: linux-stable/include/linux/serial_core.h
===================================================================
--- linux-stable.orig/include/linux/serial_core.h	2020-05-07 09:57:02.000000000 +0200
+++ linux-stable/include/linux/serial_core.h	2020-05-07 09:57:02.000000000 +0200
@@ -154,6 +154,7 @@ struct uart_port {
 
 	/* quirks must be updated while holding port mutex */
 #define UPQ_NO_TXEN_TEST	BIT(0)
+#define UPQ_DELAY_BEFORE_READ	BIT(1)
 
 	unsigned int		read_status_mask;	/* driver specific */
 	unsigned int		ignore_status_mask;	/* driver specific */
Index: linux-stable/arch/alpha/include/asm/serial.h
===================================================================
--- linux-stable.orig/arch/alpha/include/asm/serial.h	2020-02-07 11:52:16.000000000 +0100
+++ linux-stable/arch/alpha/include/asm/serial.h	2020-05-07 14:48:21.000000000 +0200
@@ -28,3 +28,8 @@
 	{ 0, BASE_BAUD, 0x2F8, 3, STD_COM_FLAGS },	/* ttyS1 */	\
 	{ 0, BASE_BAUD, 0x3E8, 4, STD_COM_FLAGS },	/* ttyS2 */	\
 	{ 0, BASE_BAUD, 0x2E8, 3, STD_COM4_FLAGS },	/* ttyS3 */
+
+#ifdef CONFIG_PCI
+extern int alpha_has_broken_serial_ports_and_needs_delay;
+#define alpha_has_broken_serial_ports_and_needs_delay	alpha_has_broken_serial_ports_and_needs_delay
+#endif