[PATCH -rt] buggy UART fix

[PATCH -rt] buggy UART fix

[Steven Rostedt]

Ingo,

I'm not sure if this is the correct fix, but it fixes a problem on one of
our boards.  The uart does't set the IIR register upon receiving an
interrupt for transmit.  Thus we get processes stuck waiting to send
data out.

This doesn't seem to be a problem on vanilla, and I'm not sure why.
Perhaps the scheduling doesn't ever let the transmit buffer get full? Well
I haven't look too much into the vanilla side.

This patch forces the processing of the interrupt even if the iir doesn't
show that there was an interrupt, iff the uart has been detected as buggy
(which our board's uart is ) and the interrupt hasn't already handled
it.

-- Steve

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Index: linux-2.6.15-rt17/drivers/serial/8250.c
===================================================================
--- linux-2.6.15-rt17.orig/drivers/serial/8250.c	2006-02-27 10:20:31.000000000 -0500
+++ linux-2.6.15-rt17/drivers/serial/8250.c	2006-02-27 10:20:53.000000000 -0500
@@ -1344,6 +1344,17 @@ static irqreturn_t serial8250_interrupt(
 				"irq%d\n", irq);
 			break;
 		}
+		/*
+		 * If we have a buggy TX line, that doesn't
+		 * notify us via iir that we need to transmit
+		 * then force the call.
+		 */
+		if (!handled && (up->bugs & UART_BUG_TXEN)) {
+			spin_lock(&up->port.lock);
+			serial8250_handle_port(up, regs);
+			spin_unlock(&up->port.lock);
+		}
+
 	} while (l != end);

 	spin_unlock(&i->lock);

Re: [PATCH -rt] buggy UART fix

[Ingo Molnar]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> Ingo,
> 
> I'm not sure if this is the correct fix, but it fixes a problem on one 
> of our boards.  The uart does't set the IIR register upon receiving an 
> interrupt for transmit.  Thus we get processes stuck waiting to send 
> data out.
> 
> This doesn't seem to be a problem on vanilla, and I'm not sure why. 
> Perhaps the scheduling doesn't ever let the transmit buffer get full? 
> Well I haven't look too much into the vanilla side.
> 
> This patch forces the processing of the interrupt even if the iir 
> doesn't show that there was an interrupt, iff the uart has been 
> detected as buggy (which our board's uart is ) and the interrupt 
> hasn't already handled it.

thx, applied.

	Ingo

Re: [PATCH -rt] buggy UART fix

[Ingo Molnar]

hm, i just got the deadlock below, most likely due to your serial fix.  
(never got this before) It's not 100% reproducible - booted the kernel a 
dozen times (with your patch applied, working on other stuff) before i 
got this deadlock.

	Ingo

==========================================
[ BUG: lock recursion deadlock detected! |
------------------------------------------
already locked:  [c1075ea0] {&port->lock}
.. ->owner: f24d48f2
.. held by:             IRQ 4: 2400 [f24d48f0,  57]
... acquired at:               serial8250_interrupt+0xe2/0x14a
-{current task's backtrace}----------------->
 [<c0103f86>] show_trace+0x18/0x1d (20)
 [<c0103fa6>] dump_stack+0x1b/0x1f (20)
 [<c013afda>] check_deadlock+0xf6/0x2b4 (24)
 [<c0130036>] task_blocks_on_lock+0x1f/0x231 (44)
 [<c02e77e9>] ___down_mutex+0x29a/0x3f3 (96)
 [<c02e8303>] _spin_lock+0x39/0x42 (24)
 [<c020f280>] serial8250_interrupt+0xe9/0x14a (36)
 [<c013cf8f>] handle_IRQ_event+0x50/0xbd (52)
 [<c013dcd1>] do_irqd+0x171/0x298 (32)
 [<c012cac3>] kthread+0xab/0xdc (36)
 [<c0100d71>] kernel_thread_helper+0x5/0xb (229810204)
---------------------------
| preempt count: 00000001 ]
| 1-level deep critical section nesting:
----------------------------------------
.. [<c02e87de>] .... _raw_spin_lock+0x12/0x26
.....[<c02e7575>] ..   ( <= ___down_mutex+0x26/0x3f3)


showing all tasks:
hm, tasklist_lock locked, retrying...  #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 ignoring it.
 locked it.
S            init:    1 [c2db78f0, 120] (not blocked)
S     migration/0:    2 [c2dbb8f0,   0] (not blocked)
S  softirq-high/0:    3 [c2dbf8f0,  98] (not blocked)
S softirq-timer/0:    4 [c2dc38f0,  98] (not blocked)
S softirq-net-tx/:    5 [c2dc78f0,  98] (not blocked)
S softirq-net-rx/:    6 [c2dcb8f0,  98] (not blocked)
S softirq-block/0:    7 [c2dcf8f0,  98] (not blocked)
S softirq-tasklet:    8 [c2dd38f0,  98] (not blocked)
S      watchdog/0:    9 [c2dd78f0,   0] (not blocked)
S       desched/0:   10 [c2ddb8f0, 105] (not blocked)
S        events/0:   11 [c2df88f0,  98] (not blocked)
S         khelper:   12 [c2dfd8f0, 111] (not blocked)
S         kthread:   13 [c2e1f8f0, 111] (not blocked)
S       kblockd/0:   15 [c2e4e8f0, 110] (not blocked)
S          kacpid:   16 [f7ea18f0, 112] (not blocked)
S           IRQ 9:   17 [f7ea88f0,  50] (not blocked)
S         pdflush:  186 [f7ec18f0, 117] (not blocked)
S         pdflush:  187 [f796f8f0, 115] (not blocked)
S           aio/0:  189 [f7ec88f0, 112] (not blocked)
S         kswapd0:  188 [f7ed18f0, 125] (not blocked)
S           IRQ 8:  772 [f7a0b8f0,  51] (not blocked)
S         kseriod:  776 [f79fb8f0, 110] (not blocked)
S          IRQ 12:  779 [f71398f0,  52] (not blocked)
S          IRQ 14:  816 [f715d8f0,  53] (not blocked)
S          IRQ 15:  817 [f7a128f0,  54] (not blocked)
S           IRQ 1:  834 [f721e8f0,  55] (not blocked)
S       kjournald:  838 [f71348f0, 115] (not blocked)
S       kjournald: 1833 [f676b8f0, 123] (not blocked)
S           udevd: 1836 [f61978f0, 112] (not blocked)
S          IRQ 16: 2114 [f650a8f0,  56] (not blocked)
S         portmap: 2214 [f666c8f0, 119] (not blocked)
S           acpid: 2247 [f60268f0, 116] (not blocked)
S            sshd: 2257 [f71f98f0, 118] (not blocked)
M         distccd: 2287 [f60068f0, 120] (not blocked)
S         distccd: 2288 [f63078f0, 125] (not blocked)
S             gpm: 2296 [f60808f0, 116] (not blocked)
S           crond: 2304 [f60b08f0, 119] (not blocked)
S         distccd: 2319 [f63c18f0, 121] (not blocked)
S         distccd: 2320 [f63348f0, 121] (not blocked)
S             xfs: 2327 [f60c78f0, 118] (not blocked)
S             atd: 2342 [f62b18f0, 119] (not blocked)
S         distccd: 2350 [f63928f0, 121] (not blocked)
S     dbus-daemon: 2351 [f63628f0, 116] (not blocked)
S            hald: 2359 [f2cd98f0, 115] (not blocked)
S         distccd: 2364 [f284e8f0, 121] (not blocked)
S hald-addon-acpi: 2365 [f2a328f0, 118] (not blocked)
S         distccd: 2373 [f2ac38f0, 121] (not blocked)
S              su: 2381 [f61228f0, 118] (not blocked)
S          agetty: 2386 [f60f38f0, 116] (not blocked)
S        mingetty: 2387 [f66708f0, 118] (not blocked)
R        mingetty: 2388 [f2b4e8f0, 118] (not blocked)
S        mingetty: 2389 [f29a38f0, 118] (not blocked)
S        mingetty: 2390 [f242c8f0, 118] (not blocked)
S        mingetty: 2391 [f24478f0, 118] (not blocked)
R        mingetty: 2392 [f24618f0, 118] (not blocked)
S            bash: 2399 [f24c38f0, 118] (not blocked)
D           IRQ 4: 2400 [f24d48f0,  57] (not blocked)
R            udev: 2403 [f25468f0, 116] (not blocked)
S            udev: 2418 [f26318f0, 117] (not blocked)
R default.hotplug: 2434 [f20008f0, 119] (not blocked)
S            udev: 2438 [f255a8f0, 116] (not blocked)
R default.hotplug: 2442 [f21e98f0, 119] (not blocked)
M           udevd: 2452 [f20f38f0, 119] (not blocked)
S            udev: 2464 [f26758f0, 118] (not blocked)
S            udev: 2472 [f20648f0, 116] (not blocked)
R 05-pam_console.: 2473 [f25428f0, 118] (not blocked)
D default.hotplug: 2475 [f24cf8f0, 118] blocked on: [c03565a4] {drivers/char/tty_io.c:133}
.. ->owner: f2b4e8f2
.. held by:          mingetty: 2388 [f2b4e8f0, 118]
... acquired at:               release_dev+0x1f1/0x671
R           egrep: 2478 [f23178f0, 118] (not blocked)
D        udevsend: 2479 [f23198f0, 113] blocked on: [c03cd2c0] {kernel/fork.c:67}
.. ->owner: f1ca38f2
.. held by:             udevd: 2482 [f1ca38f0, 116]
... acquired at:               sys_setpriority+0x4c/0x1c3
S            udev: 2480 [f24c68f0, 115] (not blocked)
R 05-pam_console.: 2481 [f27c98f0, 118] (not blocked)
R           udevd: 2482 [f1ca38f0, 116] (not blocked)

=============================================

[ turning off deadlock detection.Please report this trace. ]

Re: [PATCH -rt] buggy UART fix

[Paul Fulghum]

On Tue, 2006-02-28 at 21:40 +0100, Ingo Molnar wrote:
> hm, i just got the deadlock below, most likely due to your serial fix.  
> (never got this before) It's not 100% reproducible - booted the kernel a 
> dozen times (with your patch applied, working on other stuff) before i 
> got this deadlock.

The original patch grabs the port.lock and then
calls serial8250_handle_port() which grabs the lock
again. Just remove the spin_lock/spin_unlock below.

+ /*
+ * If we have a buggy TX line, that doesn't
+ * notify us via iir that we need to transmit
+ * then force the call.
+ */
+ if (!handled && (up->bugs & UART_BUG_TXEN)) {
+ spin_lock(&up->port.lock);
+ serial8250_handle_port(up, regs);
+ spin_unlock(&up->port.lock);
+ }

-- 
Paul Fulghum
Microgate Systems, Ltd