[PATCH 2/2 v3] alpha: add a delay before serial port read

[Mikulas Patocka <mpatocka@redhat.com>]

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

> On Wed, May 06, 2020 at 11:57:08AM -0400, Mikulas Patocka wrote:
> > 
> > 
> > On Wed, 6 May 2020, Greg Kroah-Hartman wrote:
> > 
> > > On Wed, May 06, 2020 at 11:29:29AM -0400, Mikulas Patocka wrote:
> > > > 
> > > > @@ -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
> > > 
> > > Why is a #define needed here?
> > > 
> > > You can do this same type of change without any #ifdefs in any .c files
> > > and that would be much nicer.
> > 
> > Because alpha_serial_port_needs_delay is defined only on Alpha - so we 
> > need to guard accesses to it with #ifdef CONFIG_ALPHA - otherwise the 
> > kernel wouldn't link on non-Alpha platforms.
> > 
> > Should I make it defined for all architectures?
> 
> Yes, it's not the first time we have had to do things like this :)

I've created this patch that adds a global macro/variable 
serial_port_needs_delay. I've also deleted UPQ_DELAY_BEFORE_READ and test 
serial_port_needs_delay directly in io_serial_in, so that the compiler 
will optimize it out on non-alpha architectures.

> But, there is no other way to detect this based on hardware
> signatures/types instead?  That is usually the best way to do it, right?

It's hard to detect Alpha without using '#ifdef CONFIG_ALPHA' :) The ISA 
serial port hardware is simple, so I think that you can't distinguish it 
just based on its behavior.

> thanks,
> 
> greg k-h
> 


From: Mikulas Patocka <mpatocka@redhat.com>

alpha: add a delay before serial port read

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        |    3 +++
 arch/alpha/kernel/pci.c             |    4 ++++
 drivers/tty/serial/8250/8250_port.c |    4 ++++
 include/linux/pci.h                 |    4 ++++
 4 files changed, 15 insertions(+)

Index: linux-stable/arch/alpha/include/asm/pci.h
===================================================================
--- linux-stable.orig/arch/alpha/include/asm/pci.h	2020-05-06 18:54:24.000000000 +0200
+++ linux-stable/arch/alpha/include/asm/pci.h	2020-05-06 18:54:24.000000000 +0200
@@ -97,4 +97,7 @@ 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 serial_port_needs_delay;
+#define serial_port_needs_delay	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 18:54:24.000000000 +0200
+++ linux-stable/arch/alpha/kernel/pci.c	2020-05-06 18:54:24.000000000 +0200
@@ -61,9 +61,13 @@ struct pci_controller *pci_isa_hose;
  * Quirks.
  */
 
+int serial_port_needs_delay = 0;
+EXPORT_SYMBOL(serial_port_needs_delay);
+
 static void quirk_isa_bridge(struct pci_dev *dev)
 {
 	dev->class = PCI_CLASS_BRIDGE_ISA << 8;
+	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 18:54:24.000000000 +0200
+++ linux-stable/drivers/tty/serial/8250/8250_port.c	2020-05-06 18:54:24.000000000 +0200
@@ -30,6 +30,7 @@
 #include <linux/uaccess.h>
 #include <linux/pm_runtime.h>
 #include <linux/ktime.h>
+#include <linux/pci.h>
 
 #include <asm/io.h>
 #include <asm/irq.h>
@@ -442,6 +443,9 @@ static unsigned int mem32be_serial_in(st
 
 static unsigned int io_serial_in(struct uart_port *p, int offset)
 {
+	if (serial_port_needs_delay)
+		ndelay(300);
+
 	offset = offset << p->regshift;
 	return inb(p->iobase + offset);
 }
Index: linux-stable/include/linux/pci.h
===================================================================
--- linux-stable.orig/include/linux/pci.h	2020-05-06 18:54:24.000000000 +0200
+++ linux-stable/include/linux/pci.h	2020-05-06 18:54:24.000000000 +0200
@@ -2384,6 +2384,10 @@ static inline bool pci_is_thunderbolt_at
 	return false;
 }
 
+#ifndef serial_port_needs_delay
+#define serial_port_needs_delay	0
+#endif
+
 #if defined(CONFIG_PCIEPORTBUS) || defined(CONFIG_EEH)
 void pci_uevent_ers(struct pci_dev *pdev, enum  pci_ers_result err_type);
 #endif