On Thu, Jan 14, 2021 at 10:41 AM Rafael J. Wysocki <[email protected]> wrote:
>
> From: Rafael J. Wysocki <[email protected]>
>
> When adding a new device link, device_is_dependent() is used to
> check whether or not the prospective supplier device does not
> depend on the prospective consumer one to avoid adding loops
> to the graph of device dependencies.
>
> However, device_is_dependent() does not take the ancestors of
> the target device into account, so it may not detect an existing
> reverse dependency if, for example, the parent of the target
> device depends on the device passed as its first argument.
>
> For this reason, extend device_is_dependent() to also check if
> the device passed as its first argument is an ancestor of the
> target one and return 1 if that is the case.
>
> Signed-off-by: Rafael J. Wysocki <[email protected]>
> Reported-by: Stephan Gerhold <[email protected]>
> ---
> drivers/base/core.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> Index: linux-pm/drivers/base/core.c
> ===================================================================
> --- linux-pm.orig/drivers/base/core.c
> +++ linux-pm/drivers/base/core.c
> @@ -208,6 +208,16 @@ int device_links_read_lock_held(void)
> #endif
> #endif /* !CONFIG_SRCU */
>
> +static bool device_is_ancestor(struct device *dev, struct device *target)
> +{
> + while (target->parent) {
> + target = target->parent;
> + if (dev == target)
> + return true;
> + }
> + return false;
> +}
> +
> /**
> * device_is_dependent - Check if one device depends on another one
> * @dev: Device to check dependencies for.
> @@ -221,7 +231,7 @@ int device_is_dependent(struct device *d
> struct device_link *link;
> int ret;
>
> - if (dev == target)
> + if (dev == target || device_is_ancestor(dev, target))
> return 1;
>
> ret = device_for_each_child(dev, target, device_is_dependent);
>
The code works, but it's not at all obvious what it's doing. Because,
at first glance, it's easy to mistakenly think that it's trying to
catch this case:
dev <- child1 <- child2 <- target
Maybe it's clearer if we do this check inside the loop? Something like:
if (link->consumer == target ||
device_is_ancestor(link->consumer, target))
return 1;
-Saravana
