On Fri, Jan 15, 2021 at 5:03 AM Rafael J. Wysocki <raf...@kernel.org> wrote:
>
> On Fri, Jan 15, 2021 at 11:03 AM Stephan Gerhold <step...@gerhold.net> wrote:
> >
> > Hi,
> >
> > On Thu, Jan 14, 2021 at 11:31:12AM -0800, Saravana Kannan wrote:
> > > On Thu, Jan 14, 2021 at 10:41 AM Rafael J. Wysocki <r...@rjwysocki.net>
> > > wrote:
> > > >
> > > > From: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> > > >
> > > > 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 <rafael.j.wyso...@intel.com>
> > > > Reported-by: Stephan Gerhold <step...@gerhold.net>
> > > > ---
> > > > 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);
> > > >
> > >
> >
> > Thanks for the patch, Rafael! I tested it and it seems to avoid the
> > circular device link (and therefore also the crash). FWIW:
> >
> > Tested-by: Stephan Gerhold <step...@gerhold.net>
>
> Thanks!
>
> > > 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
> > >
> >
> > Isn't this pretty much the case we are trying to catch? I have:
> >
> > 78d9000.usb <- ci_hdrc.0 <- ci_hdrc.0.ulpi <- phy-ci_hdrc.0.ulpi.0
> >
> > then something attempts to create a device link with
> > consumer = 78d9000.usb, supplier = phy-ci_hdrc.0.ulpi.0, and to check if
> > that is allowed we call device_is_dependent() with dev = 78d9000.usb,
> > target = phy-ci_hdrc.0.ulpi.0.
> >
> > Note that this case would normally be covered by the
> > device_for_each_child().
> > It's not in this case because the klist_children of 78d9000.usb
> > is updated too late.
>
> Exactly.
Stephan,
What device/driver is this? Is this a dwc3 device/driver? That driver
does some weird/incorrect stuff the last time I checked.
>
> The supplier has been initialized, which is why device_is_dependent()
> is invoked at all, but it has not been fully registered yet, so
> device_for_each_child() cannot be relied on to catch all of the
> possible dependencies.
Rafael,
Ok, I understand this additional case now.
What functions does one call to get the device to this state? AFAIR,
device_add() does both the "initialization" and adding to the
parent's children list.
I'm okay with this, but I want to make sure the driver isn't doing weird stuff.
-Saravana
