powerpc/xmon: Fallback to printk() in xmon_printf() if udbg is not setup

It is possible to configure a kernel which has xmon enabled, but has no
udbg backend to provide IO. This can make xmon rather confusing, as it
produces no output, blocks for two seconds, and then returns.

As a last resort we can instead try to printk(), which may deadlock or
otherwise crash, but tries quite hard not to.

Signed-off-by: Michael Ellerman <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/xmon/nonstdio.c

···
 void xmon_printf(const char *format, ...)
 {
 	va_list args;
-	int n;
 	static char xmon_outbuf[1024];
+	int rc, n;
 
 	va_start(args, format);
 	n = vsnprintf(xmon_outbuf, sizeof(xmon_outbuf), format, args);
 	va_end(args);
-	xmon_write(xmon_outbuf, n);
+
+	rc = xmon_write(xmon_outbuf, n);
+
+	if (n && rc == 0) {
+		/* No udbg hooks, fallback to printk() - dangerous */
+		printk(xmon_outbuf);
+	}
 }
 
 void xmon_puts(const char *str)