On Thu, Feb 23, 2006 at 10:13:50PM +0000, Martin Michlmayr wrote:
> I've tracked down now while the old 2.6.12 Debian package shut down
> correctly while no recent git does. The following simple change to
> the serial driver makes the difference for me:
>
> --- a/drivers/serial/serial_core.c~ 2006-02-23 21:58:51.000000000 +0000
> +++ b/drivers/serial/serial_core.c 2006-02-23 21:59:14.000000000 +0000
> @@ -108,7 +108,8 @@
> static void uart_tasklet_action(unsigned long data)
> {
> struct uart_state *state = (struct uart_state *)data;
> - tty_wakeup(state->info->tty);
> + if (state->info->tty)
> + tty_wakeup(state->info->tty);
> }
>
> static inline void
>
> I cannot easily check why this change was in Debian's 2.6.12 package
> nor why it's not in Linus' git. Russell, can you say whether this
> change looks obviously good to you? If not, I can dig some more and
> see why this change was in our 2.6.12 package.
This looks like a case of a fix to work around some bad behaviour in
a driver.
When serial_core calls the drivers shutdown() method, it expects that
will shut the driver up completely - no further interrupts from that
point in, no further calls from the driver into the tty or serial_core
subsystems for this port.
Once the drivers shutdown() method has returned, we ensure there are
no pending interrupts, and then kill off the tasklet.
Sometime later, we NULL state->info->tty.
Hence, if you're seeing a call into uart_tasklet_action() with a NULL
tty, that means some driver called uart_write_wakeup() _after_ its
shutdown method completed. That's a driver bug, not a serial_core
bug - as I say above, the above patch is a workaround for a buggy
driver.
Looking at the ip22 driver, it seems that if shutdown() is called for
the console port, the driver does _nothing_. That's the bug - it will
still call (eg) uart_write_wakeup() after shutdown(), which is a
violation of the assumptions (set out above).
I would not be surprised therefore if you got an oops.
What I might consider is a patch to add BUG_ON(!info->tty) in
uart_write_wakeup() to catch these cases earlier and to provide
folk with a better hint that the bug is _not_ in the core serial
driver.
--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core
