[PATCH 2/2 v2] alpha: add a delay before serial port read

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 6 May 2020, Greg Kroah-Hartman wrote:

> > 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
> 
> We really do not like #ifdef in .c files, especially ones that cause a
> coding style violation :)
> 
> Why can't you do this as a quirk for this specific chipset?  You should
> tie it to the serial port hardware type, not to the CPU type.
> 
> thanks,
> 
> greg k-h

Do you want this patch? It enables the delay based on the specific PCI-ISA 
bridge.

There is still "#if defined(CONFIG_ALPHA) && defined(CONFIG_PCI)" because 
if we want to reference a variable defined by the PCI subsystem in the 
arch/alpha tree, we must do it conditionally. If you want to get rid of 
these #ifs, please describe how.

Mikulas


From: 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+

---
 arch/alpha/include/asm/pci.h        |    2 ++
 arch/alpha/kernel/pci.c             |    4 ++++
 drivers/tty/serial/8250/8250_core.c |    7 +++++++
 drivers/tty/serial/8250/8250_port.c |    3 +++
 include/linux/serial_core.h         |    1 +
 5 files changed, 17 insertions(+)

Index: linux-stable/include/linux/serial_core.h
===================================================================
--- linux-stable.orig/include/linux/serial_core.h	2020-05-06 17:16:28.000000000 +0200
+++ linux-stable/include/linux/serial_core.h	2020-05-06 17:16:28.000000000 +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/drivers/tty/serial/8250/8250_core.c
===================================================================
--- linux-stable.orig/drivers/tty/serial/8250/8250_core.c	2020-05-06 17:16:28.000000000 +0200
+++ linux-stable/drivers/tty/serial/8250/8250_core.c	2020-05-06 17:16:28.000000000 +0200
@@ -37,6 +37,9 @@
 #ifdef CONFIG_SPARC
 #include <linux/sunserialcore.h>
 #endif
+#if defined(CONFIG_ALPHA) && defined(CONFIG_PCI)
+#include <linux/pci.h>
+#endif
 
 #include <asm/irq.h>
 
@@ -490,6 +493,10 @@ static void univ8250_rsa_support(struct
 static inline void serial8250_apply_quirks(struct uart_8250_port *up)
 {
 	up->port.quirks |= skip_txen_test ? UPQ_NO_TXEN_TEST : 0;
+#if defined(CONFIG_ALPHA) && defined(CONFIG_PCI)
+	if (alpha_serial_port_needs_delay)
+		up->port.quirks |= UPQ_DELAY_BEFORE_READ;
+#endif
 }
 
 static void __init serial8250_isa_init_ports(void)
Index: linux-stable/arch/alpha/include/asm/pci.h
===================================================================
--- linux-stable.orig/arch/alpha/include/asm/pci.h	2020-05-06 17:16:28.000000000 +0200
+++ linux-stable/arch/alpha/include/asm/pci.h	2020-05-06 17:16:28.000000000 +0200
@@ -97,4 +97,6 @@ extern void pci_adjust_legacy_attr(struc
 extern int pci_create_resource_files(struct pci_dev *dev);
 extern void pci_remove_resource_files(struct pci_dev *dev);
 
+extern int alpha_serial_port_needs_delay;
+
 #endif /* __ALPHA_PCI_H */
Index: linux-stable/arch/alpha/kernel/pci.c
===================================================================
--- linux-stable.orig/arch/alpha/kernel/pci.c	2020-05-06 17:16:28.000000000 +0200
+++ linux-stable/arch/alpha/kernel/pci.c	2020-05-06 17:16:28.000000000 +0200
@@ -61,9 +61,13 @@ struct pci_controller *pci_isa_hose;
  * Quirks.
  */
 
+int alpha_serial_port_needs_delay = 0;
+EXPORT_SYMBOL(alpha_serial_port_needs_delay);
+
 static void quirk_isa_bridge(struct pci_dev *dev)
 {
 	dev->class = PCI_CLASS_BRIDGE_ISA << 8;
+	alpha_serial_port_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	2020-05-06 17:16:28.000000000 +0200
+++ linux-stable/drivers/tty/serial/8250/8250_port.c	2020-05-06 17:16:32.000000000 +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);
 }