On Thu, Aug 29, 2019 at 2:46 AM Saravana Kannan <saravanak@xxxxxxxxxx> 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 <lkp@xxxxxxxxx>
> Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>
> ---
> .../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
