linux-kernel - Re: device_for_each_child() before device

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Message-ID: <20090415051548.GC3462@kroah.com>
Date:	Tue, 14 Apr 2009 22:15:48 -0700
From:	Greg KH <greg@...ah.com>
To:	David Vrabel <david.vrabel@....com>
Cc:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: device_for_each_child() before device_add() doesn't work

On Tue, Apr 14, 2009 at 07:16:31PM +0100, David Vrabel wrote:
> Greg KH wrote:
> > On Tue, Apr 14, 2009 at 05:54:40PM +0100, David Vrabel wrote:
> >> Calling device_for_each_child() on a device that has yet to be added
> >> (with device_add()) used to work.  UWB subsystem currently tries to do this.
> >>
> >> Since "driver core: move klist_children into private structure"[1] this
> >> no longer works (an oops occurs in device_for_each_chid()).  Is it
> >> something that ought to work? Or should the UWB subsystem be changed to
> >> not do this?
> > 
> > Why would you call device_for_each_child() if you have never added it to
> > the bus?  How would you expect that to work, as there will never be any
> > children for it?
> 
> The children of a UWB radio controller (RC) are it's neighbors.  When 
> assigning a random DevAddr to an RC we need to check that the address is 
>   unique within the beacon group.  We use device_for_each_child() here 
> to check that the generated DevAddr is not used by any of its neighbors.

So the problem is for the first device being added, right?  Can't you
just have a flag for this?

> This address must be assigned during initialization and during normal 
> operation and in the past we didn't need to special case the address 
> assignment during initialization.

Does the patch below fix the problem for you?

thanks,

greg k-h

---------------

 drivers/base/core.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1142,6 +1142,9 @@ int device_for_each_child(struct device 
 	struct device *child;
 	int error = 0;
 
+	if (!parent->p)
+		return 0;
+
 	klist_iter_init(&parent->p->klist_children, &i);
 	while ((child = next_device(&i)) && !error)
 		error = fn(child, data);
--
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