of: property: Improve cycle detection when one of the devices is never added

Consider this example where -> means LHS device is a consumer of RHS
device and indentation represents "child of" of the previous device.

Device A -> Device C

Device B -> Device A
	Device C

Without this commit:
1. Device A is added.
2. Device A is added to waiting for supplier list (Device C)
3. Device B is added
4. Device B is linked as a consumer to Device A
5. Device A doesn't probe because it's waiting for Device C to be added.
6. Device B doesn't probe because Device A hasn't probed.
7. Device C will never be added because it's parent hasn't probed.

So, Device A, B and C will be in a probe/add deadlock.

This commit detects this scenario and stops trying to create a device
link between Device A and Device C since doing so would create the
following cycle:
Device A -> Devic C -(parent)-> Device B -> Device A.

With this commit:
1. Device A is added.
3. Device B is added
4. Device B is linked as a consumer to Device A
5. Device A probes.
6. Device B probes because Device A has probed.
7. Device C is added and probed.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20200610011934.49795-3-saravanak@google.com
Signed-off-by: Rob Herring <robh@kernel.org>

diff --git a/drivers/of/property.c b/drivers/of/property.c
index 1f2086f4e7ce843344d02fca4be04352497c371c..6a5760f0d6cd63dce2353e6168ff26671c5a83c3 100644 (file)
--- a/drivers/of/property.c
+++ b/drivers/of/property.c
@@ -1014,6 +1014,30 @@ static bool of_is_ancestor_of(struct device_node *test_ancestor,
        return false;
 }
 
+/**
+ * of_get_next_parent_dev - Add device link to supplier from supplier phandle
+ * @np: device tree node
+ *
+ * Given a device tree node (@np), this function finds its closest ancestor
+ * device tree node that has a corresponding struct device.
+ *
+ * The caller of this function is expected to call put_device() on the returned
+ * device when they are done.
+ */
+static struct device *of_get_next_parent_dev(struct device_node *np)
+{
+       struct device *dev = NULL;
+
+       of_node_get(np);
+       do {
+               np = of_get_next_parent(np);
+               if (np)
+                       dev = get_dev_from_fwnode(&np->fwnode);
+       } while (np && !dev);
+       of_node_put(np);
+       return dev;
+}
+
 /**
  * of_link_to_phandle - Add device link to supplier from supplier phandle
  * @dev: consumer device
@@ -1035,10 +1059,9 @@ static bool of_is_ancestor_of(struct device_node *test_ancestor,
 static int of_link_to_phandle(struct device *dev, struct device_node *sup_np,
                              u32 dl_flags)
 {
-       struct device *sup_dev;
+       struct device *sup_dev, *sup_par_dev;
        int ret = 0;
        struct device_node *tmp_np = sup_np;
-       int is_populated;
 
        of_node_get(sup_np);
        /*
@@ -1075,16 +1098,43 @@ static int of_link_to_phandle(struct device *dev, struct device_node *sup_np,
                return -EINVAL;
        }
        sup_dev = get_dev_from_fwnode(&sup_np->fwnode);
-       is_populated = of_node_check_flag(sup_np, OF_POPULATED);
-       of_node_put(sup_np);
-       if (!sup_dev && is_populated) {
+       if (!sup_dev && of_node_check_flag(sup_np, OF_POPULATED)) {
                /* Early device without struct device. */
                dev_dbg(dev, "Not linking to %pOFP - No struct device\n",
                        sup_np);
+               of_node_put(sup_np);
                return -ENODEV;
        } else if (!sup_dev) {
-               return -EAGAIN;
+               /*
+                * DL_FLAG_SYNC_STATE_ONLY doesn't block probing and supports
+                * cycles. So cycle detection isn't necessary and shouldn't be
+                * done.
+                */
+               if (dl_flags & DL_FLAG_SYNC_STATE_ONLY) {
+                       of_node_put(sup_np);
+                       return -EAGAIN;
+               }
+
+               sup_par_dev = of_get_next_parent_dev(sup_np);
+
+               if (sup_par_dev && device_is_dependent(dev, sup_par_dev)) {
+                       /* Cyclic dependency detected, don't try to link */
+                       dev_dbg(dev, "Not linking to %pOFP - cycle detected\n",
+                               sup_np);
+                       ret = -EINVAL;
+               } else {
+                       /*
+                        * Can't check for cycles or no cycles. So let's try
+                        * again later.
+                        */
+                       ret = -EAGAIN;
+               }
+
+               of_node_put(sup_np);
+               put_device(sup_par_dev);
+               return ret;
        }
+       of_node_put(sup_np);
        if (!device_link_add(dev, sup_dev, dl_flags))
                ret = -EINVAL;
        put_device(sup_dev);