[PATCH] Re: 2.6.21-rc7-mm2 -- hvsi console driver registration failure

[Andy Whitcroft <apw@shadowen.org>]

Trying to get 2.6.21-rc7-mm2 to boot on large PPC64 seems to be a
bit of a challenge.  We have been seeing panics on boot from the
hvsi driver:

	Couldn't register hvsi console driver

Tracking this back, this seems to come from hvsi driver trying to
register itself via tty_register_driver() with a zero units.

The failure is triggered by a change in semantics for kmalloc()
between SLAB and SLUB; kmalloc(0) now returns NULL rather than an
allocation at the smallest size.  Looking at the code in question
even when the allocation succeeds we will not actually use the
memory when device->num is zero.

It is not clear to me if this is a bug in the hvsi driver in that
it should specify some units.  It seems we will try and reserve zero
devices in this case, which seems pointless.

I have tested with the patch below which seems safe to me and stops
the errors and even seems to make the console work.  But perhaps
someone with more driver fu, could verify if driver->num of zero
has any meaning and kick this to the hvsi people if not.

-apw

=== 8< ===
tty_register_driver: only allocate tty instances when defined

If device->num is zero we attempt to kmalloc() zero bytes.
When SLUB is enabled this returns a null pointer and take that as
an allocation failure and fail the device register.  Check for no
devices and avoid the allocation.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
---
diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 959a616..71c4579 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -3724,7 +3724,7 @@ int tty_register_driver(struct tty_driver *driver)
 	if (driver->flags & TTY_DRIVER_INSTALLED)
 		return 0;
 
-	if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM)) {
+	if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM) && driver->num) {
 		p = kmalloc(driver->num * 3 * sizeof(void *), GFP_KERNEL);
 		if (!p)
 			return -ENOMEM;