On Thu, Aug 29, 2019 at 2:46 AM Saravana Kannan [off-list ref] wrote:

Add device links after the devices are created (but before they are
probed) by looking at common DT bindings like clocks and
interconnects.

Automatically adding device links for functional dependencies at the
framework level provides the following benefits:

- Optimizes device probe order and avoids the useless work of
  attempting probes of devices that will not probe successfully
  (because their suppliers aren't present or haven't probed yet).

  For example, in a commonly available mobile SoC, registering just
  one consumer device's driver at an initcall level earlier than the
  supplier device's driver causes 11 failed probe attempts before the
  consumer device probes successfully. This was with a kernel with all
  the drivers statically compiled in. This problem gets a lot worse if
  all the drivers are loaded as modules without direct symbol
  dependencies.

- Supplier devices like clock providers, interconnect providers, etc
  need to keep the resources they provide active and at a particular
  state(s) during boot up even if their current set of consumers don't
  request the resource to be active. This is because the rest of the
  consumers might not have probed yet and turning off the resource
  before all the consumers have probed could lead to a hang or
  undesired user experience.

  Some frameworks (Eg: regulator) handle this today by turning off
  "unused" resources at late_initcall_sync and hoping all the devices
  have probed by then. This is not a valid assumption for systems with
  loadable modules. Other frameworks (Eg: clock) just don't handle
  this due to the lack of a clear signal for when they can turn off
  resources. This leads to downstream hacks to handle cases like this
  that can easily be solved in the upstream kernel.

  By linking devices before they are probed, we give suppliers a clear
  count of the number of dependent consumers. Once all of the
  consumers are active, the suppliers can turn off the unused
  resources without making assumptions about the number of consumers.

By default we just add device-links to track "driver presence" (probe
succeeded) of the supplier device. If any other functionality provided
by device-links are needed, it is left to the consumer/supplier
devices to change the link when they probe.

kbuild test robot reported clang error about missing const
Reported-by: kbuild test robot <redacted>
Signed-off-by: Saravana Kannan <redacted>
---
 .../admin-guide/kernel-parameters.rst         |   1 +
 .../admin-guide/kernel-parameters.txt         |   6 +
 drivers/of/property.c                         | 241 ++++++++++++++++++
 3 files changed, 248 insertions(+)

+static int of_link_to_phandle(struct device *dev, struct device_node *sup_np)
+{
+       struct platform_device *sup_dev;
+       u32 dl_flags = DL_FLAG_AUTOPROBE_CONSUMER;
+       int ret = 0;
+       struct device_node *tmp_np = sup_np;
+
+       of_node_get(sup_np);
+       /*
+        * Find the device node that contains the supplier phandle.  It may be
+        * @sup_np or it may be an ancestor of @sup_np.
+        */
+       while (sup_np && !of_find_property(sup_np, "compatible", NULL))
+               sup_np = of_get_next_parent(sup_np);
+       if (!sup_np) {
+               dev_dbg(dev, "Not linking to %pOFP - No device\n", tmp_np);
+               return -ENODEV;
+       }
+
+       /*
+        * Don't allow linking a device node as a consumer of one of its
+        * descendant nodes. By definition, a child node can't be a functional
+        * dependency for the parent node.
+        */
+       if (!of_is_ancestor_of(dev->of_node, sup_np)) {
+               dev_dbg(dev, "Not linking to %pOFP - is descendant\n", sup_np);
+               of_node_put(sup_np);
+               return -EINVAL;
+       }
+       sup_dev = of_find_device_by_node(sup_np);

What if the supplier isn't a platform_device? A regulator supply is
quite likely not.

Rob