lkml.org 
[lkml]   [2016]   [Sep]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC/RFT][PATCH v2 2/7] driver core: Functional dependencies tracking support
    On Wed, Sep 14, 2016 at 03:21:27AM +0200, Rafael J. Wysocki wrote:
    > On Sunday, September 11, 2016 10:43:36 PM Lukas Wunner wrote:
    > > On Sun, Sep 11, 2016 at 03:40:58PM +0200, Lukas Wunner wrote:
    > > > 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.
    > >
    > > Oh I'm grasping only now, you want to emit a WARN for *every*
    > > infringing child/consumer. That could lead to a WARN flood if
    > > a developer accidentally does something really dumb, like linking
    > > the PCI root to some PCI endpoint device, but fair enough.
    > >
    > > The point about linking a parent to a child still stands however.
    > > I think a simple way to check this is to just add
    > >
    > > if (WARN_ON(dev == target))
    > > return 1;
    > >
    > > at the top of the function, because when someone tries to link
    > > a parent to a child, when recursing from the parent downward
    > > one will eventually hit that child. This will also prevent
    > > someone from linking a device to itself.
    >
    > I actually would prefer to make it impossible to link a parent to
    > a child at all.

    Which is precisely what the code snippet above does.

    Thanks,

    Lukas

    \
     
     \ /
      Last update: 2016-09-17 09:59    [W:7.342 / U:0.008 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site