[PATCH 2.6.26] SERIAL DRIVER: Handle Multiple consecutive sysrq from the serial

[Eran Liberty <liberty@extricom.com>]

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Dear Penguins,

Let me start of by saying my particular hardware must be buggy in some 
way. When I issue a sysrq (Ctrl A+ F from minicom) I get a lot of sysrq 
triggers.

I have worked around the problem and I think this workaround is a viable 
patch even for platforms which do not exhibit this peculiar behavior.

upon getting numerous interrupts which request sysrq the function 
uart_handle_break in include/linux/serial_core.h is hit multiple times.
The current code which looks like this:

static inline int uart_handle_break(struct uart_port *port)
 {
        struct uart_info *info = port->info;
 #ifdef SUPPORT_SYSRQ
        if (port->cons && port->cons->index == port->line) {
               if (!port->sysrq) {
                       port->sysrq = jiffies + HZ*5;
                       return 1;
               }
               port->sysrq = 0;
        }
 #endif
        if (port->flags & UPF_SAK)
                do_SAK(info->tty);
       return 0;
}

Will basicly toggle port->sysrq between a timeout value and zero. If you 
are lucky this penguin rullet will stop on timeout and the next 
character hit will trigger the sysrq in the function 
"uart_handle_sysrq_char". But if you are not so lucky the last sysrq 
interupt will toggle port->sysrq to zero and the next char hit will be 
ignored (not trigger sysrq).

The suggested patch will do the next few things:

1. "port->sysrq" is now the time when the last sysrq was triggered and 
not the timeout for the the next char
2. Stamped "port->sysrq" every time there is a sysrq rather then toggled 
it up and down.
3. Always continue to consider UPF_SAK.
4. "port->sysrq" is toggled back to zero only in uart_handle_break() and 
only if the a char has been accepted after the sysrq timeout (5 sec)
5. uart_handle_break() will ignore extra chars received in super human 
speed after the last sysrq (0.01 sec)

Liberty

Signed-off-by: Eran Liberty <liberty@extricom.org>
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Index: include/linux/serial_core.h
===================================================================
--- include/linux/serial_core.h	(revision 119)
+++ include/linux/serial_core.h	(revision 120)
@@ -447,8 +447,8 @@
 uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
 {
 #ifdef SUPPORT_SYSRQ
-	if (port->sysrq) {
-		if (ch && time_before(jiffies, port->sysrq)) {
+	if (port->sysrq && time_after(jiffies, port->sysrq + (unsigned long)(HZ*0.01))) {
+		if (ch && time_before(jiffies, port->sysrq + HZ*5)) {
 			handle_sysrq(ch, port->info ? port->info->tty : NULL);
 			port->sysrq = 0;
 			return 1;
@@ -467,19 +467,17 @@
  */
 static inline int uart_handle_break(struct uart_port *port)
 {
+	int ret = 0;
 	struct uart_info *info = port->info;
 #ifdef SUPPORT_SYSRQ
 	if (port->cons && port->cons->index == port->line) {
-		if (!port->sysrq) {
-			port->sysrq = jiffies + HZ*5;
-			return 1;
-		}
-		port->sysrq = 0;
+		port->sysrq = jiffies;
+		ret = 1;
 	}
 #endif
 	if (port->flags & UPF_SAK)
 		do_SAK(info->tty);
-	return 0;
+	return ret;
 }
 
 /**