Re: [PATCH v10 2/7] of: property: Add functional dependency link from DT bindings

[Rob Herring]

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