serial: Emulate break using control characters

Currently the magic SysRq functions are accessed by sending a break.
Unfortunately some networked serial proxies makes is difficult to send
a break meaning SysRq functions cannot be reached. We avoid this problem
by allowing the (fairly unlikely) sequence of ^B^R^K characters to
emulate a real break.

This approach is very nearly as robust as normal sysrq/break handling
because all trigger recognition happens during interrupt handling however
to emulate a break we must enter the ISR four times (instead of twice) and
manage an extra byte of state.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index e2894b23efb6..9563d5315cf1 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -372,6 +372,21 @@ config MAGIC_SYSRQ_DEFAULT_ENABLE
 	  This may be set to 1 or 0 to enable or disable them all, or
 	  to a bitmask as described in Documentation/sysrq.txt.
 
+config MAGIC_SYSRQ_BREAK_EMULATION
+	bool "Enable magic SysRq serial break emulation"
+	depends on MAGIC_SYSRQ && SERIAL_CORE_CONSOLE
+	default n
+	help
+	  If you say Y here, then you can use the character sequence ^B^R^K
+	  to simulate a BREAK on the serial console. This is useful if for
+	  some reason you cannot send a BREAK to your console's serial port.
+	  For example, if you have a serial device server that cannot
+	  send a BREAK. Enabling this feature can delay the delivery of
+	  characters to the TTY because the ^B and a subsequent ^R will be
+	  delayed until we know what the next character is.
+
+	  If unsure, say N.
+
 config DEBUG_KERNEL
 	bool "Kernel debugging"
 	help