Commit 9deaa53ac7fa373623123aa4f18828dd62292b1a

Authored by Paul Gortmaker
Committed by Greg Kroah-Hartman
1 parent 86b21199fc

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>

Showing 5 changed files with 73 additions and 0 deletions Side-by-side Diff

arch/powerpc/kernel/legacy_serial.c
... ... @@ -441,6 +441,9 @@
441 441 return;
442 442  
443 443 port->irq = virq;
  444 +
  445 + if (of_device_is_compatible(np, "fsl,ns16550"))
  446 + port->handle_irq = fsl8250_handle_irq;
444 447 }
445 448  
446 449 static void __init fixup_port_pio(int index,
drivers/tty/serial/8250_fsl.c
  1 +#include <linux/serial_reg.h>
  2 +#include <linux/serial_8250.h>
  3 +
  4 +#include "8250.h"
  5 +
  6 +/*
  7 + * Freescale 16550 UART "driver", Copyright (C) 2011 Paul Gortmaker.
  8 + *
  9 + * This program is free software; you can redistribute it and/or modify
  10 + * it under the terms of the GNU General Public License version 2 as
  11 + * published by the Free Software Foundation.
  12 + *
  13 + * This isn't a full driver; it just provides an alternate IRQ
  14 + * handler to deal with an errata. Everything else is just
  15 + * using the bog standard 8250 support.
  16 + *
  17 + * We follow code flow of serial8250_default_handle_irq() but add
  18 + * a check for a break and insert a dummy read on the Rx for the
  19 + * immediately following IRQ event.
  20 + *
  21 + * We re-use the already existing "bug handling" lsr_saved_flags
  22 + * field to carry the "what we just did" information from the one
  23 + * IRQ event to the next one.
  24 + */
  25 +
  26 +int fsl8250_handle_irq(struct uart_port *port)
  27 +{
  28 + unsigned char lsr, orig_lsr;
  29 + unsigned long flags;
  30 + unsigned int iir;
  31 + struct uart_8250_port *up =
  32 + container_of(port, struct uart_8250_port, port);
  33 +
  34 + spin_lock_irqsave(&up->port.lock, flags);
  35 +
  36 + iir = port->serial_in(port, UART_IIR);
  37 + if (iir & UART_IIR_NO_INT) {
  38 + spin_unlock_irqrestore(&up->port.lock, flags);
  39 + return 0;
  40 + }
  41 +
  42 + /* This is the WAR; if last event was BRK, then read and return */
  43 + if (unlikely(up->lsr_saved_flags & UART_LSR_BI)) {
  44 + up->lsr_saved_flags &= ~UART_LSR_BI;
  45 + port->serial_in(port, UART_RX);
  46 + spin_unlock_irqrestore(&up->port.lock, flags);
  47 + return 1;
  48 + }
  49 +
  50 + lsr = orig_lsr = up->port.serial_in(&up->port, UART_LSR);
  51 +
  52 + if (lsr & (UART_LSR_DR | UART_LSR_BI))
  53 + lsr = serial8250_rx_chars(up, lsr);
  54 +
  55 + serial8250_modem_status(up);
  56 +
  57 + if (lsr & UART_LSR_THRE)
  58 + serial8250_tx_chars(up);
  59 +
  60 + up->lsr_saved_flags = orig_lsr;
  61 + spin_unlock_irqrestore(&up->port.lock, flags);
  62 + return 1;
  63 +}
drivers/tty/serial/Kconfig
... ... @@ -97,6 +97,11 @@
97 97 This builds standard PNP serial support. You may be able to
98 98 disable this feature if you only need legacy serial support.
99 99  
  100 +config SERIAL_8250_FSL
  101 + bool
  102 + depends on SERIAL_8250 && PPC
  103 + default PPC
  104 +
100 105 config SERIAL_8250_HP300
101 106 tristate
102 107 depends on SERIAL_8250 && HP300
drivers/tty/serial/Makefile
... ... @@ -28,6 +28,7 @@
28 28 obj-$(CONFIG_SERIAL_8250_EXAR_ST16C554) += 8250_exar_st16c554.o
29 29 obj-$(CONFIG_SERIAL_8250_HUB6) += 8250_hub6.o
30 30 obj-$(CONFIG_SERIAL_8250_MCA) += 8250_mca.o
  31 +obj-$(CONFIG_SERIAL_8250_FSL) += 8250_fsl.o
31 32 obj-$(CONFIG_SERIAL_8250_DW) += 8250_dw.o
32 33 obj-$(CONFIG_SERIAL_AMBA_PL010) += amba-pl010.o
33 34 obj-$(CONFIG_SERIAL_AMBA_PL011) += amba-pl011.o
include/linux/serial_8250.h
... ... @@ -82,6 +82,7 @@
82 82 struct ktermios *termios, struct ktermios *old);
83 83 extern void serial8250_do_pm(struct uart_port *port, unsigned int state,
84 84 unsigned int oldstate);
  85 +extern int fsl8250_handle_irq(struct uart_port *port);
85 86 int serial8250_handle_irq(struct uart_port *port, unsigned int iir);
86 87 unsigned char serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr);
87 88 void serial8250_tx_chars(struct uart_8250_port *up);