hvc_console: Better kernel console support

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

hvc_console has two methods to instanciate the consoles.

hvc_instanciate is meant to be called at early boot, while hvc_alloc is
called for more dynamically probed objects.

Currently, it only deals with adding kernel consoles in the former case,
which means for example that if a console only uses dynamic probing, it
will never be usable as a kernel console even when specifying
console=hvc0 explicitly, which could be considered annoying...

More specifically, on pseries, we only do the early instanciate for the
console currently used by the firmware, so if you have your firmware
configured to go to a video card, for example, you cannot get your
kernel console, oops messages, etc... on your serial port or hypervisor
console, which would be handy to deal with oopses.

This fixes it by checking if hvc_console.flags & CON_ENABLED is set when
registering a new dynamic console, and if not, redo the index check and
re-register the console if the index matches, allowing console=hvcN to
work.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---
diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 2d691eb..f1d4d96 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -245,6 +245,20 @@ static void hvc_port_destruct(struct tty_port *port)
 	kfree(hp);
 }
 
+static void hvc_check_console(int index)
+{
+	/* Already enabled, bail out */
+	if (hvc_console.flags & CON_ENABLED)
+		return;
+
+ 	/* If this index is what the user requested, then register
+	 * now (setup won't fail at this point).  It's ok to just
+	 * call register again if previously .setup failed.
+	 */
+	if (index == hvc_console.index)
+		register_console(&hvc_console);
+}
+
 /*
  * hvc_instantiate() is an early console discovery method which locates
  * consoles * prior to the vio subsystem discovering them.  Hotplugged
@@ -275,12 +289,8 @@ int hvc_instantiate(uint32_t vtermno, int index, const struct hv_ops *ops)
 	if (last_hvc < index)
 		last_hvc = index;
 
-	/* if this index is what the user requested, then register
-	 * now (setup won't fail at this point).  It's ok to just
-	 * call register again if previously .setup failed.
-	 */
-	if (index == hvc_console.index)
-		register_console(&hvc_console);
+	/* check if we need to re-register the kernel console */
+	hvc_check_console(index);
 
 	return 0;
 }
@@ -858,10 +868,15 @@ struct hvc_struct *hvc_alloc(uint32_t vtermno, int data,
 		i = ++last_hvc;
 
 	hp->index = i;
+	cons_ops[i] = ops;
+	vtermnos[i] = vtermno;
 
 	list_add_tail(&(hp->next), &hvc_structs);
 	spin_unlock(&hvc_structs_lock);
 
+	/* check if we need to re-register the kernel console */
+	hvc_check_console(i);
+
 	return hp;
 }
 EXPORT_SYMBOL_GPL(hvc_alloc);
@@ -874,8 +889,12 @@ int hvc_remove(struct hvc_struct *hp)
 	tty = tty_port_tty_get(&hp->port);
 
 	spin_lock_irqsave(&hp->lock, flags);
-	if (hp->index < MAX_NR_HVC_CONSOLES)
+	if (hp->index < MAX_NR_HVC_CONSOLES) {
+		console_lock();
 		vtermnos[hp->index] = -1;
+		cons_ops[hp->index] = NULL;
+		console_unlock();
+	}
 
 	/* Don't whack hp->irq because tty_hangup() will need to free the irq. */