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SubjectRe: [PATCH] 8250 UART backup timer
From
Date

Ping. Any other comments on this? Thanks,

Alex

On Tue, 2006-06-27 at 13:04 -0600, Alex Williamson wrote:
> The patch below works 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).
>
> To solve this problem, a backup timer is introduced that runs
> alongside the standard interrupt driven mechanism. This timer wakes up
> periodically, checks for a hang condition and gets characters moving
> again. This backup mechanism is only enabled if the UART is detected as
> having this problem, so systems without these UARTs will have no
> additional overhead.
>
> This version of the patch incorporates previous comments from Pavel
> and removes races in the bug detection code. The test is now done
> before the irq linking to prevent races with interrupt handler clearing
> the THRE interrupt. Short delays and syncs are also added to ensure the
> device is able to update register state before the result is tested.
> Comments? Thanks,
>
> Alex
>
> Signed-off-by: Alex Williamson <alex.williamson@hp.com>
> ---
>
> diff -r e3554576c29a drivers/serial/8250.c
> --- a/drivers/serial/8250.c Sun Jun 18 02:00:07 2006 +0000
> +++ b/drivers/serial/8250.c Tue Jun 27 12:27:03 2006 -0600
> @@ -353,6 +353,23 @@ serial_out(struct uart_8250_port *up, in
> }
> }
>
> +static void
> +serial_out_sync(struct uart_8250_port *up, int offset, int value)
> +{
> + switch (up->port.iotype) {
> + case UPIO_MEM:
> + case UPIO_MEM32:
> +#ifdef CONFIG_SERIAL_8250_AU1X00
> + case UPIO_AU:
> +#endif
> + serial_out(up, offset, value);
> + (void)serial_in(up, UART_LCR); /* safe, no side-effects */
> + break;
> + default:
> + serial_out(up, offset, value);
> + }
> +}
> +
> /*
> * We used to support using pause I/O for certain machines. We
> * haven't supported this for a while, but just in case it's badly
> @@ -1436,6 +1453,13 @@ static void serial_unlink_irq_chain(stru
> serial_do_unlink(i, up);
> }
>
> +/* Base timer interval for polling */
> +static inline int
> +poll_timeout(int timeout)
> +{
> + return timeout > 6 ? (timeout / 2 - 2) : 1;
> +}
> +
> /*
> * This function is used to handle ports that do not have an
> * interrupt. This doesn't work very well for 16450's, but gives
> @@ -1445,16 +1469,52 @@ static void serial8250_timeout(unsigned
> static void serial8250_timeout(unsigned long data)
> {
> struct uart_8250_port *up = (struct uart_8250_port *)data;
> - unsigned int timeout;
> unsigned int iir;
>
> iir = serial_in(up, UART_IIR);
> if (!(iir & UART_IIR_NO_INT))
> serial8250_handle_port(up, NULL);
>
> - timeout = up->port.timeout;
> - timeout = timeout > 6 ? (timeout / 2 - 2) : 1;
> - mod_timer(&up->timer, jiffies + timeout);
> + mod_timer(&up->timer, jiffies + poll_timeout(up->port.timeout));
> +}
> +
> +static void serial8250_backup_timeout(unsigned long data)
> +{
> + struct uart_8250_port *up = (struct uart_8250_port *)data;
> + unsigned int iir, ier = 0;
> +
> + /*
> + * Must disable interrupts or else we risk racing with the interrupt
> + * based handler.
> + */
> + 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))
> + serial8250_handle_port(up, NULL);
> +
> + if (is_real_interrupt(up->port.irq))
> + serial_out(up, UART_IER, ier);
> +
> + /* Standard timer interval plus 0.2s to keep the port running */
> + mod_timer(&up->timer, jiffies + poll_timeout(up->port.timeout) + HZ/5);
> }
>
> static unsigned int serial8250_tx_empty(struct uart_port *port)
> @@ -1523,6 +1583,36 @@ static void serial8250_break_ctl(struct
> up->lcr &= ~UART_LCR_SBC;
> serial_out(up, UART_LCR, up->lcr);
> spin_unlock_irqrestore(&up->port.lock, flags);
> +}
> +
> +#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, int bits)
> +{
> + unsigned int status, timeout = 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 (--timeout == 0)
> + break;
> + udelay(1);
> + } while ((status & bits) != bits);
> +
> + /* Wait up to 1s for flow control if necessary */
> + if (up->port.flags & UPF_CONS_FLOW) {
> + timeout = 1000000;
> + while (--timeout &&
> + ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
> + udelay(1);
> + }
> }
>
> static int serial8250_startup(struct uart_port *port)
> @@ -1598,18 +1688,50 @@ static int serial8250_startup(struct uar
> serial_outp(up, UART_LCR, 0);
> }
>
> + if (is_real_interrupt(up->port.irq)) {
> + /*
> + * 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. Delays are necessary to
> + * allow register changes to become visible.
> + */
> + spin_lock_irqsave(&up->port.lock, flags);
> +
> + wait_for_xmitr(up, UART_LSR_THRE);
> + serial_out_sync(up, UART_IER, UART_IER_THRI);
> + udelay(1); /* allow THRE to set */
> + (void)serial_in(up, UART_IIR);
> + serial_out(up, UART_IER, 0);
> + serial_out_sync(up, UART_IER, UART_IER_THRI);
> + udelay(1); /* allow a working UART time to re-assert THRE */
> + iir = serial_in(up, UART_IIR);
> + serial_out(up, UART_IER, 0);
> +
> + spin_unlock_irqrestore(&up->port.lock, flags);
> +
> + /*
> + * If the interrupt is not reasserted, setup a timer to
> + * kick the UART on a regular basis.
> + */
> + if (iir & UART_IIR_NO_INT) {
> + pr_debug("ttyS%d - using backup timer\n", port->line);
> + up->timer.function = serial8250_backup_timeout;
> + up->timer.data = (unsigned long)up;
> + mod_timer(&up->timer, jiffies +
> + poll_timeout(up->port.timeout) + HZ/5);
> + }
> + }
> +
> /*
> * If the "interrupt" for this port doesn't correspond with any
> * hardware interrupt, we use a timer-based system. The original
> * driver used to do this with IRQ0.
> */
> if (!is_real_interrupt(up->port.irq)) {
> - unsigned int timeout = up->port.timeout;
> -
> - timeout = timeout > 6 ? (timeout / 2 - 2) : 1;
> -
> up->timer.data = (unsigned long)up;
> - mod_timer(&up->timer, jiffies + timeout);
> + mod_timer(&up->timer, jiffies + poll_timeout(up->port.timeout));
> } else {
> retval = serial_link_irq_chain(up);
> if (retval)
> @@ -1725,9 +1847,9 @@ 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);
> }
>
> @@ -2187,36 +2309,6 @@ 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, int bits)
> -{
> - 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 & bits) != bits);
> -
> - /* 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 void serial8250_console_putchar(struct uart_port *port, int ch)
> {
> struct uart_8250_port *up = (struct uart_8250_port *)port;
>
>
> -
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>
--
Alex Williamson HP Open Source & Linux Org.

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