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devuart: allow serial console on late detected uarts

pci uarts are detected late and usually do not contain
the console= parameter logic.

for these, we can just enable them when devuart is reset,
and replay the boot messages once enabled.

this is usefull as it allows us to use these uarts for
kernel debugging in interrupt context.

--- a/sys/src/9/port/devuart.c

+++ b/sys/src/9/port/devuart.c

@@ -240,9 +240,30 @@

 		uart[i] = p;

 		p->dev = i;

+

+		/*

+		 * enable serial console for uarts detected

+		 * late during boot (like pci card).

+		 */

+		if(consuart == nil){

+			char *s, *options;

+

+			if((s = getconf("console")) != nil

+			&& strtoul(s, &options, 0) == i

+			&& options > s){

+				p->console = 1;

+				if(*options != '\0')

+					uartctl(p, options);

+			}

+		}

+

 		if(p->console || p->special){

 			if(uartenable(p) != nil){

 				if(p->console){

+					if(consuart == nil){

+						consuart = p;

+						uartputs(kmesg.buf, kmesg.n);

+					}

 					serialoq = p->oq;

 				}

 				p->opens++;