[Bugme-janitors] [Bug 7002] New: modem control line handling in 8250/serial_core suspend code broken

[bugme-daemon at bugzilla.kernel.org]

http://bugzilla.kernel.org/show_bug.cgi?id=7002

           Summary: modem control line handling in 8250/serial_core suspend
                    code broken
    Kernel Version: 2.6.18-rc3
            Status: NEW
          Severity: normal
             Owner: power-management_other at kernel-bugs.osdl.org
         Submitter: u.luckas at road-gmbh.de


Hardware Environment: x86, arm (pxa)
Problem Description:
there is a bug in recent kernel's suspend code for serial drivers, at least 
for 8250 and pxa, probably others though.

When suspending (e.g. apm --suspend) a machine, the serial_core suspend code 
tries to disable modem control lines (including RTS):
http://sosdg.org/~coywolf/lxr/source/drivers/serial/serial_core.c?v=2.6;a=arm#L1926
This is needed to stop the other side of a serial link from sending any data 
after our serial port has been disabled.

Now in line 1937 ops->shutdown(port) is called. For 8250 this points to 
serial8250_shutdown, for pxa to serial_pxa_shutdown. Both call 
xxx__set_mctrl(&up->port, up->port.mctrl) which will restore the old control 
line values and reverse the intended effect.

I am not quite sure about the right approach to fixing this. Should 
uart_suspend_port set port.mctrl to 0 after saveing it's old value? Or should 
serial8250_shutdown simply set port.mctrl to 0?

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