Re: [PATCH 5/6] OF: Utility helper functions for dynamic nodes

[Ioan Nicu <ioan.nicu.ext@nsn.com>]

Hi Pantelis,

On Mon, Jun 23, 2014 at 07:57:24PM +0300, ext Pantelis Antoniou wrote:
> Hi Alexander,
> 
> On Jun 23, 2014, at 7:26 PM, Alexander Sverdlin wrote:
> 
> > Hello Pantelis!
> > 
> > On 22/06/14 11:40, ext Pantelis Antoniou wrote:
> >> Introduce helper functions for working with the live DT tree,
> >> all of them related to dynamically adding/removing nodes and
> >> properties.
> >> 
> >> __of_copy_property() copies a property dynamically
> >> __of_create_empty_node() creates an empty node
> >> 
> >> Bug fix about prop->len == 0 by Ionut Nicu <ioan.nicu.ext@nsn.com>
> > 
> > Are you sure about this? (see below...)
> > 

Alexander is right, my fix was lost even though it's mentioned in this patch.

> >> Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
> >> ---
> 
> [snip]
> >> +
> >> +	if (prop->length > 0) {
> >        ^^^^^^^^^^^^^^^^^^^^^
> > Seems, that length==0 case will still produce value==NULL results,
> > which will brake some checks in the kernel... Or am I missing something in
> > the new version?
> > 
> 
> prop->value will be set to NULL, and length will be set to zero (kzalloc).
> This is a normal zero length property.
> 
> I don't know of any place in the kernel accessing the value if prop->length==0
> 

We have a simple use case. We have an overlay which adds an interrupt controller.
If you look in drivers/of/irq.c, in of_irq_parse_raw():

[...]
	/* Now start the actual "proper" walk of the interrupt tree */
	while (ipar != NULL) {
		/* Now check if cursor is an interrupt-controller and if it is
		 * then we are done
		 */
		if (of_get_property(ipar, "interrupt-controller", NULL) !=
				NULL) {
			pr_debug(" -> got it !\n");
			return 0;
		}
[...]

A node is identified as an interrupt controller if it has a zero-length property
called "interrupt-controller" but with a non-NULL value.

My proposed fix for this was to remove the if () condition. propn->value will be
allocated with kmalloc(0) which returns ZERO_SIZE_PTR which is != NULL.


> 
> [snip]
> 
> >> +
> >> #endif /* _LINUX_OF_H */
> >> 
> > 
> > -- 
> > Best regards,
> > Alexander Sverdlin.
> 
> Regards
> 
> -- Pantelis
> 

Regards,
Ionut Nicu