[PATCH] nubus: switch to dynamic root device

[PATCH] nubus: switch to dynamic root device

[Johan Hovold]

Driver core expects devices to be dynamically allocated and will, for
example, complain loudly if a device that lacks a release function is
ever freed.

Use root_device_register() to allocate and register the root device
instead of open coding using a static device.

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/nubus/nubus.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/drivers/nubus/nubus.c b/drivers/nubus/nubus.c
index 559dce302d06..40ce4991c356 100644
--- a/drivers/nubus/nubus.c
+++ b/drivers/nubus/nubus.c
@@ -41,9 +41,7 @@ module_param_named(populate_procfs, nubus_populate_procfs, bool, 0);
 
 LIST_HEAD(nubus_func_rsrcs);
 
-static struct device nubus_parent = {
-	.init_name	= "nubus",
-};
+static struct device *nubus_parent;
 
 /* Meaning of "bytelanes":
 
@@ -833,7 +831,7 @@ static void __init nubus_add_board(int slot, int bytelanes)
 		list_add_tail(&fres->list, &nubus_func_rsrcs);
 	}
 
-	if (nubus_device_register(&nubus_parent, board))
+	if (nubus_device_register(nubus_parent, board))
 		put_device(&board->dev);
 }
 
@@ -880,18 +878,17 @@ static void __init nubus_scan_bus(void)
 
 static int __init nubus_init(void)
 {
-	int err;
-
 	if (!MACH_IS_MAC)
 		return 0;
 
 	nubus_proc_init();
-	err = device_register(&nubus_parent);
-	if (err) {
-		put_device(&nubus_parent);
-		return err;
-	}
+
+	nubus_parent = root_device_register("nubus");
+	if (IS_ERR(nubus_parent))
+		return PTR_ERR(nubus_parent);
+
 	nubus_scan_bus();
+
 	return 0;
 }
 
-- 
2.53.0

Re: [PATCH] nubus: switch to dynamic root device

[Finn Thain]

On Fri, 24 Apr 2026, Johan Hovold wrote:

> Driver core expects devices to be dynamically allocated and will, for
> example, complain loudly if a device that lacks a release function is
> ever freed.
> 

Yes, in drivers/base/core.c, there is a warning in device_release().

WARN(1, KERN_ERR "Device '%s' does not have a release() 
	function, it is broken and must be fixed. See 
	Documentation/core-api/kobject.rst.\n",

But there's no way for the refcount for the nubus parent device to reach 
zero that I can see. Did I miss something?

> Use root_device_register() to allocate and register the root device
> instead of open coding using a static device.
> 

Well, dynamic allocation makes sense for busses that might be instantiated 
more than once. But I don't know of any hardware like that. The nubus 
parent device is a singleton.

I suppose I could see some benefit to converting CONFIG_NUBUS into a 
tristate. Maybe the module link in root_device_register() would become 
applicable if someone wanted to do that conversion.

Re: [PATCH] nubus: switch to dynamic root device

[Johan Hovold]

On Sat, Apr 25, 2026 at 02:07:10PM +1000, Finn Thain wrote:
> On Fri, 24 Apr 2026, Johan Hovold wrote:
> 
> > Driver core expects devices to be dynamically allocated and will, for
> > example, complain loudly if a device that lacks a release function is
> > ever freed.
> > 
> 
> Yes, in drivers/base/core.c, there is a warning in device_release().
> 
> WARN(1, KERN_ERR "Device '%s' does not have a release() 
> 	function, it is broken and must be fixed. See 
> 	Documentation/core-api/kobject.rst.\n",
> 
> But there's no way for the refcount for the nubus parent device to reach 
> zero that I can see. Did I miss something?

It will if registration ever fails (e.g. due to fault injection or name
collision):

	err = device_register(&nubus_parent);
	if (err) {
		put_device(&nubus_parent);
		return err;
	}

Johan

Re: [PATCH] nubus: switch to dynamic root device

[Finn Thain]

On Mon, 27 Apr 2026, Johan Hovold wrote:

> On Sat, Apr 25, 2026 at 02:07:10PM +1000, Finn Thain wrote:
> > On Fri, 24 Apr 2026, Johan Hovold wrote:
> > 
> > > Driver core expects devices to be dynamically allocated and will, 
> > > for example, complain loudly if a device that lacks a release 
> > > function is ever freed.
> > > 
> > 
> > Yes, in drivers/base/core.c, there is a warning in device_release().
> > 
> > WARN(1, KERN_ERR "Device '%s' does not have a release()
> > 	function, it is broken and must be fixed. See 
> > 	Documentation/core-api/kobject.rst.\n",
> > 
> > But there's no way for the refcount for the nubus parent device to 
> > reach zero that I can see. Did I miss something?
> 
> It will if registration ever fails (e.g. due to fault injection or name 
> collision):
> 
> 	err = device_register(&nubus_parent);
> 	if (err) {
> 		put_device(&nubus_parent);
> 		return err;
> 	}
> 

In that situation the kernel error message would say the device "is broken 
and must be fixed" when it was deliberately broken by fault injection.
I think the commit log should state that the aim of your patch is to avoid 
that error message for that use-case.
Aside from that quibble, the patch looks fine to me.

Acked-by: Finn Thain <fthain@linux-m68k.org>