serial: 8250: Use canary to restart console after suspend
authorPeter Hurley <peter@hurleysoftware.com>
Thu, 22 Jan 2015 17:24:30 +0000 (12:24 -0500)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Mon, 2 Feb 2015 18:11:27 +0000 (10:11 -0800)
commit4516d50aabedbe5ae334155193e4d35c02390d9a
tree19bcf6aba42e65701008dce683481dce7008fef9
parent06a4c710673184f5c750bdb2f8579e0ae1cc252c
serial: 8250: Use canary to restart console after suspend

When using no_console_suspend, the serial console may be powered off
anyway during system sleep. Upon resume, the port may be in its default
power-on state, but is expected to continue console i/o before the device
has received its pm callback. The resultant garbage i/o can cause all
kinds of havoc on the remote end.

Use the scratch register as a canary to discover if the console
has been powered-off. Write a non-zero value to the scratch register
at port suspend and reprogram the port before any console i/o if the
scratch register != canary before port resume.

This workaround is disabled for omap_8250 (which uses different divisor
programming).

Credit to Doug Anderson <dianders@chromium.org> for the idea of using
the scratch register canary to discover port power-down.

Cc: Doug Anderson <dianders@chromium.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/tty/serial/8250/8250_core.c
include/linux/serial_8250.h