serial: add irq handler for Freescale 16550 errata.

Sending a break on the SOC UARTs found in some MPC83xx/85xx/86xx
chips seems to cause a short lived IRQ storm (/proc/interrupts
typically shows somewhere between 300 and 1500 events).  Unfortunately
this renders SysRQ over the serial console completely inoperable.

The suggested workaround in the errata is to read the Rx register,
wait one character period, and then read the Rx register again.
We achieve this by tracking the old LSR value, and on the subsequent
interrupt event after a break, we don't read LSR, instead we just
read the RBR again and return immediately.

The "fsl,ns16550" is used in the compatible field of the serial
device to mark UARTs known to have this issue.

Thanks to Scott Wood for providing the errata data which led to
a much cleaner fix.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

3 files changed, 69 insertions, 0 deletions

diff --git a/drivers/tty/serial/8250_fsl.c b/drivers/tty/serial/8250_fsl.c
new file mode 100644
index 000000000000..f4d3c47b88e8
--- /dev/null
+++ b/drivers/tty/serial/8250_fsl.c
@@ -0,0 +1,63 @@
+#include <linux/serial_reg.h>
+#include <linux/serial_8250.h>
+
+#include "8250.h"
+
+/*
+ * Freescale 16550 UART "driver", Copyright (C) 2011 Paul Gortmaker.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This isn't a full driver; it just provides an alternate IRQ
+ * handler to deal with an errata.  Everything else is just
+ * using the bog standard 8250 support.
+ *
+ * We follow code flow of serial8250_default_handle_irq() but add
+ * a check for a break and insert a dummy read on the Rx for the
+ * immediately following IRQ event.
+ *
+ * We re-use the already existing "bug handling" lsr_saved_flags
+ * field to carry the "what we just did" information from the one
+ * IRQ event to the next one.
+ */
+
+int fsl8250_handle_irq(struct uart_port *port)
+{
+        unsigned char lsr, orig_lsr;
+        unsigned long flags;
+        unsigned int iir;
+        struct uart_8250_port *up =
+                container_of(port, struct uart_8250_port, port);
+
+        spin_lock_irqsave(&up->port.lock, flags);
+
+        iir = port->serial_in(port, UART_IIR);
+        if (iir & UART_IIR_NO_INT) {
+                spin_unlock_irqrestore(&up->port.lock, flags);
+                return 0;
+        }
+
+        /* This is the WAR; if last event was BRK, then read and return */
+        if (unlikely(up->lsr_saved_flags & UART_LSR_BI)) {
+                up->lsr_saved_flags &= ~UART_LSR_BI;
+                port->serial_in(port, UART_RX);
+                spin_unlock_irqrestore(&up->port.lock, flags);
+                return 1;
+        }
+
+        lsr = orig_lsr = up->port.serial_in(&up->port, UART_LSR);
+
+        if (lsr & (UART_LSR_DR | UART_LSR_BI))
+                lsr = serial8250_rx_chars(up, lsr);
+
+        serial8250_modem_status(up);
+
+        if (lsr & UART_LSR_THRE)
+                serial8250_tx_chars(up);
+
+        up->lsr_saved_flags = orig_lsr;
+        spin_unlock_irqrestore(&up->port.lock, flags);
+        return 1;
+}
diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index 705d2dc39c9a..fee9e04f42e7 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig
@@ -97,6 +97,11 @@ config SERIAL_8250_PNP
           This builds standard PNP serial support. You may be able to
           disable this feature if you only need legacy serial support.
 
+config SERIAL_8250_FSL
+        bool
+        depends on SERIAL_8250 && PPC
+        default PPC
+
 config SERIAL_8250_HP300
         tristate
         depends on SERIAL_8250 && HP300
diff --git a/drivers/tty/serial/Makefile b/drivers/tty/serial/Makefile
index af57089ddb67..75eadb8d7178 100644
--- a/drivers/tty/serial/Makefile
+++ b/drivers/tty/serial/Makefile
@@ -28,6 +28,7 @@ obj-$(CONFIG_SERIAL_8250_BOCA) += 8250_boca.o
 obj-$(CONFIG_SERIAL_8250_EXAR_ST16C554) += 8250_exar_st16c554.o
 obj-$(CONFIG_SERIAL_8250_HUB6) += 8250_hub6.o
 obj-$(CONFIG_SERIAL_8250_MCA) += 8250_mca.o
+obj-$(CONFIG_SERIAL_8250_FSL) += 8250_fsl.o
 obj-$(CONFIG_SERIAL_8250_DW) += 8250_dw.o
 obj-$(CONFIG_SERIAL_AMBA_PL010) += amba-pl010.o
 obj-$(CONFIG_SERIAL_AMBA_PL011) += amba-pl011.o