ARM: iMX: lockup with irqs disabled in drivers/tty/imx.c
Michael McTernan
Michael.McTernan.2001 at cs.bris.ac.uk
Sun May 20 07:04:27 EDT 2012
Hi All,
I've found a lockup (interrupts disabled, infinite loop) in
drivers/tty/imx.c. This is on i.MX53 (Karo module), using the Karo BSP,
but having checked the arm-soc git tree, I think the bug is here too.
The Linaro tree(s) look the same.
I see the problem when the UART is setup to not have rtscts, so
port->have_rtscts = 0 in the driver. Additionally there is nothing
hanging on the serial port (in my case the peripheral isn't yet powered
up) so the rx line is low.
I can then trigger the lockup by simply booting the unit and issuing
"echo > /dev/ttymxc1". The same can be done on the other ports too.
Once locked, magic sys-req appears not to work, pinging the target is
dead etc...
The sequence of events in the kernel goes something like this:
1) imx_startup()
2) imx_int()
\-> imx_rxint()
|-> URXD0 returns 0xd800
\-> tty_insert_flip_char(tty, 0, TTY_BREAK)
...
3) imx_start_tx()
\-> imx_transmit_buffer()
Writes '^' '@'
4) imx_set_termios()
\-> hangs when waiting for USR2_TXDC
The problem is that UCR2_IRTS isn't yet set (that comes later in
imx_set_termios()), so the transmitter is waiting for the pin to be
asserted before it can serialise out from its FIFO and set the USR2_TXDC
bit. Since CTS/RTS will never come on this port, we end up spinning in
the following bit of code in imx_set_termios():948, having earlier
called spin_lock_irqsave():
while ( !(readl(sport->port.membase + USR2) & USR2_TXDC))
barrier();
The tx was a surprise to me; it looks like the receiver sees the low rx
pin as a break which then, I think, causes a local echo sending back
'^@', as per the following stack:
(imx_start_tx+0x60/0x12c) from (__uart_start+0x44/0x48)
(__uart_start+0x44/0x48) from (uart_start+0x20/0x4c)
(uart_start+0x20/0x4c) from (process_echoes+0x25c/0x260)
(process_echoes+0x25c/0x260) from (n_tty_receive_buf+0xc90/0xf04)
(n_tty_receive_buf+0xc90/0xf04) from (flush_to_ldisc+0xfc/0x1b0)
(flush_to_ldisc+0xfc/0x1b0) from (process_one_work+0x1fc/0x330)
(process_one_work+0x1fc/0x330) from (worker_thread+0x1d0/0x2f8)
(worker_thread+0x1d0/0x2f8) from (kthread+0x7c/0x84)
(kthread+0x7c/0x84) from (kernel_thread_exit+0x0/0x8)
Had imx_set_termios() been called prior to the imx_transmit_buffer(),
UCR2_IRTS would have been set and disaster avoided.
So one workaround is to set UCR2_IRTS in imx_startup() as per the patch
on the end of this email. This still wouldn't cater for other cases
where RTS/CTS is in use but not asserted by a peripheral (or the pin
mux/pad isn't correctly setup/routed), or that termios() is called while
data is blocked waiting for CTS/RTS. The drain loop should probably
implement a timeout and WARN(), but I'm not sure if it should also use a
completion and interrupt to be correct - other serial drivers don't
appear to do this though?
Anyway, the lockup is nasty, so hopefully can be triaged?
Regards,
Mike
Signed-off-by: Michael McTernan <Michael.McTernan.2001 at cs.bris.ac.uk>
diff -Naurp orig/imx.c fixed/imx.c
--- orig/imx.c 2012-05-20 10:37:20.320041462 +0100
+++ fixed/imx.c 2012-05-20 11:18:28.512657377 +0100
@@ -741,6 +741,9 @@ static int imx_startup(struct uart_port
writel(temp, sport->port.membase + UCR1);
temp = readl(sport->port.membase + UCR2);
+ if (!sport->have_rtscts) {
+ temp |= UCR2_IRTS;
+ }
temp |= (UCR2_RXEN | UCR2_TXEN);
writel(temp, sport->port.membase + UCR2);
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