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trap: add probeaddr() to check for missing uart

--- a/sys/src/9/omap/fns.h

+++ b/sys/src/9/omap/fns.h

@@ -12,7 +12,8 @@

 int cas(long *, long, long);

 int tas(void *);

-void evenaddr(uintptr va);

+void evenaddr(uintptr a);

+int probeaddr(uintptr a);

 void procrestore(Proc *);

 void procsave(Proc *);

 void procsetup(Proc *);

--- a/sys/src/9/omap/trap.c

+++ b/sys/src/9/omap/trap.c

@@ -9,6 +9,9 @@

 #include "tos.h"

 #include "ureg.h"

+static volatile int probing;

+static volatile int probed;

+

 void

 callwithureg(void (*f) (Ureg *))

 {

@@ -447,6 +450,12 @@

 	case PsrMdabt:

 		ureg->pc -= 8;

+		if(probing && !user) {

+			ureg->pc += 4;

+			probed = 0;

+			goto out;

+		}

+

 		traparm(ureg, getdfsr(), getdfar());

 		break;

@@ -477,6 +486,7 @@

 		break;

 	}

+out:

 	splhi();

 	if(user) {

 		if(up->procctl || up->nnote)

@@ -574,4 +584,26 @@

 	kexit(ureg);

 	splhi();

+}

+

+int

+probeaddr(uintptr a)

+{

+	static Lock l;

+	volatile ulong v;

+

+	ilock(&l);

+	probing = 1;

+	probed = 1;

+

+	v = *(ulong*)a;

+

+	coherence();

+	if(!probed) {

+		iunlock(&l);

+		return 0;

+	}

+

+	iunlock(&l);

+	return 1;

 }