On 02/23/2016 11:38 PM, David Rivshin (Allworx) wrote:

On Tue, 23 Feb 2016 18:45:17 +0000
Mark Rutland [off-list ref] wrote:

quoted

On Tue, Feb 23, 2016 at 01:17:25PM -0500, David Rivshin (Allworx) wrote:

quoted

From: David Rivshin <redacted>
+static int is31fl32xx_parse_child_dt(const struct device *dev,
+				     const struct device_node *child,
+				     struct is31fl32xx_led_data *led_data)
+{
+	struct led_classdev *cdev = &led_data->cdev;
+	int ret = 0;
+	u32 reg;
+
+	cdev->name = of_get_property(child, "label", NULL) ? : child->name;

We really shouldn't be spreading of_get_property into more drivers. It's
generally not what you want, and doesn't do appropriate sanity checking.

Sorry, I copied the basic DT parsing from some other led driver (leds-pwm,
I think), and did not check if there was a better way.

quoted

Use of_property_read_string, though you may need to copy the result.

Will do.

As far as copying the result, this is an area I'm fuzzy on. If I
understand correctly the device_nodes are reference counted, and
of_property_read_string() returns a pointer to the data inside the
device_node. So it seems I'd either need to hold onto a reference,
or make a copy of the string.

devm_kstrdup() seems like it would be an easy way to go, although
maybe not the most efficient.

Using of_node_get() would seem to be more efficient, but then what
would be the right way to release that reference on remove()?
  - Does that already happen in some infrastructure when using DT
    probing?
  - Is there a devm_*() helper function that should be used?
  - Or would it be best to just keep a pointer to the device_node so
    that of_node_put() can be called on remove()?

Also, I have yet to find an example of a driver which either copies
the string or does an of_node_get() on the node. In fact just by a
count of files, it seems very few have any hope of doing either:

$ find drivers/ -name '*.c' -exec grep of_property_read_string -l {} + \
| wc -l
197
$ find drivers/ -name '*.c' -exec grep of_property_read_string -l {} + \
| xargs grep -l of_node_get | wc -l
15
$ find drivers/ -name '*.c' -exec grep of_property_read_string -l {} + \
| xargs grep -l strdup | wc -l
11

So I'm very much hoping that there's some infrastructure automatically
handling the appropriate reference counting, so that the nodes stay
around (at least) as long as the driver is instantiated...

quoted

quoted

+
+	ret = of_property_read_u32(child, "reg", &reg);
+	if (ret || reg < 1 || reg > led_data->priv->cdef->channels) {
+		dev_err(dev,
+			"Child node %s does not have a valid reg property\n",
+			child->name);
+		return -EINVAL;
+	}
+	led_data->channel = reg;
+
+	cdev->default_trigger = of_get_property(child, "linux,default-trigger",
+						NULL);

Likewise.

Will take care of this same as above.

DT nodes refcounting is enabled only when CONFIG_OF_DYNAMIC is defined,
and does matter in case Device Tree overlays are used. DT people -
please correct me if I'm wrong.

If this use case is not valid for this driver, then we can rely
on the pointer obtained with of_property_read_string,
I've been doing empirical tests and child node refcount is greater
than 0 throughout whole driver lifespan.

-- 
Best regards,
Jacek Anaszewski