Re: [PATCH v3 1/8] Input: synaptics-rmi4: Use of_get_child_by_name() instead of of_find_node_by_name()

[Andrew Duggan]

On 07/18/2016 07:48 AM, Benjamin Tissoires wrote:
> On Jul 13 2016 or thereabouts, Andrew Duggan wrote:
> > Calling of_find_node_by_name() assumes that the caller has incremented
> > the refcount of the of_node being passed in. Currently, the caller is
> > not incrementing the refcount of the of_node which results in the node
> > being prematurely freed when of_find_node_by_name() calls of_node_put()
> > on it. Instead use of_get_child_by_name() which does not call put on the
> > of_node.
> There are 2 other differences in using of_get_child_by_name() in place
> of of_find_node_by_name(). One is that now we are following the OF tree
> while the spinlock is not held. I think it's fine in our case. The
> other difference is that the returned of_node has not been called
> of_node_get() on it. I am not 100% sure, but I think it might be good to
> call of_node_get() on the of node here, and in remove call
> of_node_put(), just to be sure we don't use the of_node while it has
> been freed.

The comment for of_get_child_by_name() says that it returns an of_node 
with the refcount incremented (drivers/of/base.c:717). Also, that the 
caller needs to call of_node_put() when finished with it. I take that to 
mean that the of_node_get() has been done for me by 
of_get_child_by_name(). Then rmi_unregister_function() calls 
of_node_put() when unregistered the function device undoing 
of_get_child_by_name()'s increment of the refcount.

Unless I am missing something I think the current implementation is 
correct. Hopefully, it is since this patch has already landed in Linus's 
tree.

Andrew

> Cheers,
> Benjamin
>
> > Signed-off-by: Andrew Duggan <aduggan@xxxxxxxxxxxxx>
> > ---
> >   drivers/input/rmi4/rmi_bus.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/input/rmi4/rmi_bus.c b/drivers/input/rmi4/rmi_bus.c
> > index b368b05..253df96 100644
> > --- a/drivers/input/rmi4/rmi_bus.c
> > +++ b/drivers/input/rmi4/rmi_bus.c
> > @@ -157,11 +157,11 @@ static int rmi_function_match(struct device *dev, struct device_driver *drv)
> >   static void rmi_function_of_probe(struct rmi_function *fn)
> >   {
> >   	char of_name[9];
> > +	struct device_node *node = fn->rmi_dev->xport->dev->of_node;
> >   
> >   	snprintf(of_name, sizeof(of_name), "rmi4-f%02x",
> >   		fn->fd.function_number);
> > -	fn->dev.of_node = of_find_node_by_name(
> > -				fn->rmi_dev->xport->dev->of_node, of_name);
> > +	fn->dev.of_node = of_get_child_by_name(node, of_name);
> >   }
> >   #else
> >   static inline void rmi_function_of_probe(struct rmi_function *fn)
> > --
> > 2.5.0