From: Alex Williamson Work around a minor bug found in the UART of the remote management card used in many HP ia64 and parisc servers (aka the Diva UARTs). The problem is that the UART does not reassert the THRE interrupt if it has been previously cleared and the IIR THRI bit is re-enabled. This can produce a very annoying failure mode when used as a serial console, allowing a boot/reboot to hang indefinitely until an RX interrupt kicks it into working again (ie. an unattended reboot could stall). Paul Bame submitted a complete workaround for 2.4 kernels a few years ago - http://lkml.org/lkml/2003/5/6/203. The problem has not been as prevalent on the 2.6 serial driver, thus the solution below is simplified to only insert a backup timeout to kick the UART when it gets into trouble. This runs alongside the normal interrupt driven UART code and has a longer period that the standard polling driver to reduce CPU overhead. The detection test should be safe for all UARTs and the backup timer can easily be extended to include other UARTs with similar disorders. Signed-off-by: Alex Williamson Cc: Russell King Signed-off-by: Andrew Morton --- drivers/serial/8250.c | 141 ++++++++++++++++++++++++++++++++++++++------------ 1 files changed, 108 insertions(+), 33 deletions(-) diff -puN drivers/serial/8250.c~backup-timer-for-uarts-that-lose-interrupts-take-3 drivers/serial/8250.c --- 25/drivers/serial/8250.c~backup-timer-for-uarts-that-lose-interrupts-take-3 Thu Nov 17 14:58:49 2005 +++ 25-akpm/drivers/serial/8250.c Thu Nov 17 14:58:49 2005 @@ -1438,6 +1438,50 @@ static void serial8250_timeout(unsigned mod_timer(&up->timer, jiffies + timeout); } +static void serial8250_backup_timeout(unsigned long data) +{ + struct uart_8250_port *up = (struct uart_8250_port *)data; + unsigned int iir, ier = 0; + unsigned int timeout = up->port.timeout; + + /* + * Must disable interrupts or else we risk triggering an interrupt + * and getting dead locked by the port.lock. + */ + if (is_real_interrupt(up->port.irq)) { + ier = serial_in(up, UART_IER); + serial_out(up, UART_IER, 0); + } + + iir = serial_in(up, UART_IIR); + + /* + * This should be a safe test for anyone who doesn't trust the + * IIR bits on their UART, but it's specifically designed for + * the "Diva" UART used on the management processor on many HP + * ia64 and parisc boxes. + */ + if ((iir & UART_IIR_NO_INT) && (up->ier & UART_IER_THRI) && + (!uart_circ_empty(&up->port.info->xmit) || up->port.x_char) && + (serial_in(up, UART_LSR) & UART_LSR_THRE)) { + iir &= ~(UART_IIR_ID | UART_IIR_NO_INT); + iir |= UART_IIR_THRI; + } + + if (!(iir & UART_IIR_NO_INT)) { + spin_lock(&up->port.lock); + serial8250_handle_port(up, NULL); + spin_unlock(&up->port.lock); + } + + if (is_real_interrupt(up->port.irq)) + serial_out(up, UART_IER, ier); + + timeout = timeout > 6 ? (timeout / 2 - 2) : 1; + + mod_timer(&up->timer, jiffies + (timeout * 100)); +} + static unsigned int serial8250_tx_empty(struct uart_port *port) { struct uart_8250_port *up = (struct uart_8250_port *)port; @@ -1506,6 +1550,36 @@ static void serial8250_break_ctl(struct spin_unlock_irqrestore(&up->port.lock, flags); } +#define BOTH_EMPTY (UART_LSR_TEMT | UART_LSR_THRE) + +/* + * Wait for transmitter & holding register to empty + */ +static void wait_for_xmitr(struct uart_8250_port *up) +{ + unsigned int status, tmout = 10000; + + /* Wait up to 10ms for the character(s) to be sent. */ + do { + status = serial_in(up, UART_LSR); + + if (status & UART_LSR_BI) + up->lsr_break_flag = UART_LSR_BI; + + if (--tmout == 0) + break; + udelay(1); + } while ((status & BOTH_EMPTY) != BOTH_EMPTY); + + /* Wait up to 1s for flow control if necessary */ + if (up->port.flags & UPF_CONS_FLOW) { + tmout = 1000000; + while (--tmout && + ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0)) + udelay(1); + } +} + static int serial8250_startup(struct uart_port *port) { struct uart_8250_port *up = (struct uart_8250_port *)port; @@ -1634,6 +1708,37 @@ static int serial8250_startup(struct uar up->bugs &= ~UART_BUG_TXEN; } + if (is_real_interrupt(up->port.irq) && !timer_pending(&up->timer)) { + /* + * Test for UARTs that do not reassert THRE when the + * transmitter is idle and the interrupt has already + * been cleared. Real 16550s should always reassert + * this interrupt whenever the transmitter is idle and + * the interrupt is enabled. + */ + wait_for_xmitr(up); + serial_outp(up, UART_IER, UART_IER_THRI); + (void)serial_in(up, UART_IIR); + serial_outp(up, UART_IER, 0); + serial_outp(up, UART_IER, UART_IER_THRI); + iir = serial_in(up, UART_IIR); + serial_outp(up, UART_IER, 0); + + /* + * If the interrupt is not reasserted, setup a timer to + * kick the UART on a regular basis. + */ + if (iir & UART_IIR_NO_INT) { + unsigned int timeout = up->port.timeout; + + pr_debug("ttyS%d - using backup timer\n", port->line); + timeout = timeout > 6 ? (timeout / 2 - 2) : 1; + up->timer.function = serial8250_backup_timeout; + up->timer.data = (unsigned long)up; + mod_timer(&up->timer, jiffies + (timeout * 100)); + } + } + spin_unlock_irqrestore(&up->port.lock, flags); /* @@ -1706,9 +1811,9 @@ static void serial8250_shutdown(struct u */ (void) serial_in(up, UART_RX); - if (!is_real_interrupt(up->port.irq)) - del_timer_sync(&up->timer); - else + del_timer_sync(&up->timer); + up->timer.function = serial8250_timeout; + if (is_real_interrupt(up->port.irq)) serial_unlink_irq_chain(up); } @@ -2169,36 +2274,6 @@ serial8250_register_ports(struct uart_dr #ifdef CONFIG_SERIAL_8250_CONSOLE -#define BOTH_EMPTY (UART_LSR_TEMT | UART_LSR_THRE) - -/* - * Wait for transmitter & holding register to empty - */ -static inline void wait_for_xmitr(struct uart_8250_port *up) -{ - unsigned int status, tmout = 10000; - - /* Wait up to 10ms for the character(s) to be sent. */ - do { - status = serial_in(up, UART_LSR); - - if (status & UART_LSR_BI) - up->lsr_break_flag = UART_LSR_BI; - - if (--tmout == 0) - break; - udelay(1); - } while ((status & BOTH_EMPTY) != BOTH_EMPTY); - - /* Wait up to 1s for flow control if necessary */ - if (up->port.flags & UPF_CONS_FLOW) { - tmout = 1000000; - while (--tmout && - ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0)) - udelay(1); - } -} - /* * Print a string to the serial port trying not to disturb * any possible real use of the port... _