I am getting kernel panic (NULL pointer dereference) on boot, with kernel
compiled with CONFIG_SND_MPU401_UART=y, on machine which does not have
this piece of hardware.

I have traced the problem down to
sound/drivers/mpu401/mpu401.c:snd_mpu401_probe() returning EINVAL, when
either port or IRQ parameters are not specified.

In such case, the drivers/base/bus.c:bus_attach_device() does not perform
klist_add_tail() call, but rather sets dev->is_registered to 0. This flag
is however not checked by the driver, so later on, when alsa_card_mpu401_init()
is called and platform_device_register_simple() fails, the following callchain
happens, causing NULL pointer dereference: alsa_card_mpu401_init() ->
platform_device_unregister() -> platform_device_del() -> device_del() ->
bus_remove_device() -> klist_del().

Proper solution is returning ENODEV from the ->probe() routine, which will
be correctly handled then by the rest of the device-driver attaching subsystem
(namely the retval check in bus_attach_device()). The following patch fixes
the problem, please apply.

Patch against current Linus' git tree.

Signed-off-by: Jiri Kosina <jikos@jikos.cz>

--- a/sound/drivers/mpu401/mpu401.c
+++ b/sound/drivers/mpu401/mpu401.c
@@ -104,11 +104,11 @@ static int __devinit snd_mpu401_probe(st
 
 	if (port[dev] == SNDRV_AUTO_PORT) {
 		snd_printk(KERN_ERR "specify port\n");
-		return -EINVAL;
+		return -ENODEV;
 	}
 	if (irq[dev] == SNDRV_AUTO_IRQ) {
 		snd_printk(KERN_ERR "specify or disable IRQ\n");
-		return -EINVAL;
+		return -ENODEV;
 	}
 	err = snd_mpu401_create(dev, &card);
 	if (err < 0)

Jaroslav Kysela pointed out that better solution is

--- a/drivers/base/bus.c
+++ b/drivers/base/bus.c
@@ -428,8 +428,10 @@ void bus_remove_device(struct device * d
 		sysfs_remove_link(&dev->kobj, "bus");
 		sysfs_remove_link(&dev->bus->devices.kobj, dev->bus_id);
 		device_remove_attrs(dev->bus, dev);
-		dev->is_registered = 0;
-		klist_del(&dev->knode_bus);
+		if (dev->is_registered) {
+			dev->is_registered = 0;
+			klist_del(&dev->knode_bus);
+		}
 		pr_debug("bus %s: remove device %s\n", dev->bus->name, dev->bus_id);
 		device_release_driver(dev);
		put_bus(dev->bus);

Acked-by: Jiri Kosina <jikos@jikos.cz>