[3.11.y.z extended stable] Patch "serial: 8250: enable UART_BUG_NOMSR for Tegra" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    serial: 8250: enable UART_BUG_NOMSR for Tegra

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 5fc2c6db5506f187d7a9a3618306d6161e51fa74 Mon Sep 17 00:00:00 2001
From: Stephen Warren <swarren at nvidia.com>
Date: Tue, 7 Jan 2014 15:00:12 -0700
Subject: serial: 8250: enable UART_BUG_NOMSR for Tegra

commit 3685f19e07802ec4207b52465c408f185b66490e upstream.

Tegra chips have 4 or 5 identical UART modules embedded. UARTs C..E have
their MODEM-control signals tied off to a static state. However UARTs A
and B can optionally route those signals to/from package pins, depending
on the exact pinmux configuration.

When these signals are not routed to package pins, false interrupts may
trigger either temporarily, or permanently, all while not showing up in
the IIR; it will read as NO_INT. This will eventually lead to the UART
IRQ being disabled due to unhandled interrupts. When this happens, the
kernel may print e.g.:

    irq 68: nobody cared (try booting with the "irqpoll" option)

In order to prevent this, enable UART_BUG_NOMSR. This prevents
UART_IER_MSI from being enabled, which prevents the false interrupts
from triggering.

In practice, this is not needed under any of the following conditions:

* On Tegra chips after Tegra30, since the HW bug has apparently been
  fixed.

* On UARTs C..E since their MODEM control signals are tied to the correct
  static state which doesn't trigger the issue.

* On UARTs A..B if the MODEM control signals are routed out to package
  pins, since they will then carry valid signals.

However, we ignore these exceptions for now, since they are only relevant
if a board actually hooks up more than a 4-wire UART, and no currently
supported board does this. If we ever support a board that does, we can
refine the algorithm that enables UART_BUG_NOMSR to take those exceptions
into account, and/or read a flag from DT/... that indicates that the
board has hooked up and pinmux'd more than a 4-wire UART.

Reported-by: Olof Johansson <olof at lixom.net> # autotester
Signed-off-by: Stephen Warren <swarren at nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/tty/serial/8250/8250_core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 86c00b1c..d28d7af 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -2670,6 +2670,10 @@ static void serial8250_config_port(struct uart_port *port, int flags)
 	if (port->type == PORT_16550A && port->iotype == UPIO_AU)
 		up->bugs |= UART_BUG_NOMSR;

+	/* HW bugs may trigger IRQ while IIR == NO_INT */
+	if (port->type == PORT_TEGRA)
+		up->bugs |= UART_BUG_NOMSR;
+
 	if (port->type != PORT_UNKNOWN && flags & UART_CONFIG_IRQ)
 		autoconfig_irq(up);

--
1.8.3.2