backup timer for UARTs that lose interrupts (take 2)
PATCH 1/1 backup timer for UARTs that lose interrupts (take 2)
Hi Russell,
I've revised the patch for this backup timer idea based on your
comments. Hopefully the restoring of the timer function to the default serial8250_timeout is more obvious now. I also re-ordered some of the conditions around the bug test to add further clarity. Please let me know if you see any further issues with this patch. Patch below is against 2.6.14-mm2. Here's the original patch description message:
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). 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.
Please apply. Thanks,
Alex
Signature · 115 lines
Signed-off-by: Alex Williamson <[email protected]> diff -r 855fd5ef0839 drivers/serial/8250.c --- a/drivers/serial/8250.c Fri Nov 11 04:45:10 2005 +++ b/drivers/serial/8250.c Wed Nov 16 09:06:35 2005 @@ -1425,6 +1425,50 @@ 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; @@ -1497,6 +1541,8 @@ int kgdb_irq = -1; EXPORT_SYMBOL(kgdb_irq); #endif + +static inline void wait_for_xmitr(struct uart_8250_port *up); static int serial8250_startup(struct uart_port *port) { @@ -1631,6 +1677,37 @@ 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 when ever 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); /* @@ -1703,9 +1780,9 @@ */ (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); }
Could you _please_ not put inline patches after the signature mark ("-- "). In my mailer (mozilla) this causes the patch to be greyed out and, more importantly, NOT included in a reply. This, in turn, makes it hard to comment on details in the patch.
Thanks
George
Quoted 30 lines · Alex Williamson
Hi Russell,
I've revised the patch for this backup timer idea based on your
comments. Hopefully the restoring of the timer function to the defaultserial8250_timeoutis more obvious now. I also re-ordered some of the conditions around the bug test to add further clarity. Please let me know if you see any further issues with this patch. Patch below is against 2.6.14-mm2. Here's the original patch description message: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). 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.Please apply. Thanks,
Alex
Signature · 2 lines
George Anzinger [email protected] HRT (High-res-timers): http://sourceforge.net/projects/high-res-timers/
Quoted 3 lines · George Anzinger
Could you _please_ not put inline patches after the signature mark ("-- "). In my mailer (mozilla) this causes the patch to be greyed out and, more importantly, NOT included in a reply. This, in turn, makes it hard to comment on details in the patch.
Would it help if Alex used the documented canonical patch format, using "---" instead of "-- "? I sorta doubt it.
BTW, George, please hit Return/Enter around column 70+... Thanks.
---
~Randy
Quoted 11 lines · Randy.Dunlap
Quoted 3 lines · George Anzinger
Could you _please_ not put inline patches after the signature mark ("-- "). In my mailer (mozilla) this causes the patch to be greyed out and, more importantly, NOT included in a reply. This, in turn, makes it hard to comment on details in the patch.
Would it help if Alex used the documented canonical patch format, using "---" instead of "-- "? I sorta doubt it.
I suspect it would. The "-- \n" is a documented signature mark (took me a while to find this out) and I have found no way to make mozilla do anything different with it. I don't have any documentation on "---" but it surly is not a signature mark and I suspect most mailers will not attach any signifigance to it.
Quoted 2 lines
BTW, George, please hit Return/Enter around column 70+...
Sorry, some how my wrap line length moved up to 100. Fixed now.
Signature · 2 lines
George Anzinger [email protected] HRT (High-res-timers): http://sourceforge.net/projects/high-res-timers/
Quoted 3 lines · George Anzinger
Could you _please_ not put inline patches after the signature mark ("-- "). In my mailer (mozilla) this causes the patch to be greyed out and, more importantly, NOT included in a reply. This, in turn, makes it hard to comment on details in the patch.
Sorry, I'll avoid that in the future. With Evolution, If you
highlight part of the message before you reply it will include the highlighted portions in the reply and disregard the sig mark. Do you have specific comments about this patch? I think this patch is short enough to cut-n-paste, so I'll refrain from re-posting unless you have objections. Thanks,
Alex
Signature · 1 line
Alex Williamson HP Linux & Open Source Lab