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SubjectRe: [RFC/RFT][PATCH v2 2/7] driver core: Functional dependencies tracking support
On Thu, Sep 08, 2016 at 11:27:45PM +0200, Rafael J. Wysocki wrote:
> +/**
> + * device_is_dependent - Check if one device depends on another one
> + * @dev: Device to check dependencies for.
> + * @target: Device to check against.
> + *
> + * Check if @dev or any device dependent on it (its child or its consumer etc)
> + * depends on @target. Return 1 if that is the case or 0 otherwise.
> + */
> +static int device_is_dependent(struct device *dev, void *target)
> +{
> + struct device_link *link;
> + int ret;
> +
> + ret = device_for_each_child(dev, target, device_is_dependent);
> + list_for_each_entry(link, &dev->links_to_consumers, s_node) {
> + if (WARN_ON(link->consumer == target))
> + return 1;
> +
> + ret = ret || device_is_dependent(link->consumer, target);
> + }
> + return ret;
> +}

What happens if someone tries to add a device link from a parent
(as the consumer) to a child (as a supplier)? You're only checking
if target is a consumer of dev, for full correctness you'd also have
to check if target is a parent of dev. (Or grandparent, or great-
grandparent, ... you need to walk the tree up to the root.)


The function can be sped up by returning immediately if a match
is found instead of continuing searching and accumulating the
result in ret, i.e.:

if (device_for_each_child(dev, target, device_is_dependent))
return 1;

and in the list_for_each_entry block:

if (device_is_dependent(link->consumer, target))
return 1;

Then at the end of the function "return 0".


I'd move the WARN_ON() to the single invocation of this function in
device_link_add(), that way it's possible to use the function as a
helper elsewhere should the need arise.

Thanks,

Lukas

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