hvc_xen: add earlycon support

Introduce EARLYCON support in hvc_xen, useful for early debugging on arm
and arm64, where xen early_printk is not available.

It is different from xenboot_write_console on x86 in two ways:

- it does not return if !xen_pv_domain(), not only because ARM guests
  are xen_hvm_domain(), but also because we want to capture all the
  early boot messages, before xen support is discovered
- it does not try to print to the domU console at all, because xen
  support will only be discovered at a later point

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/tty/hvc/hvc_xen.c

@@ -25,6 +25,7 @@
 #include <linux/init.h>
 #include <linux/types.h>
 #include <linux/list.h>
+#include <linux/serial_core.h>
 
 #include <asm/io.h>
 #include <asm/xen/hypervisor.h>
@@ -664,3 +665,18 @@ void xen_raw_printk(const char *fmt, ...)
 
 	xen_raw_console_write(buf);
 }
+
+static void xenboot_earlycon_write(struct console *console,
+				  const char *string,
+				  unsigned len)
+{
+	dom0_write_console(0, string, len);
+}
+
+static int __init xenboot_earlycon_setup(struct earlycon_device *device,
+					    const char *opt)
+{
+	device->con->write = xenboot_earlycon_write;
+	return 0;
+}
+EARLYCON_DECLARE(xenboot, xenboot_earlycon_setup);