Re: [PATCH v5 02/11] of/platform: Add functional dependency link from DT bindings

[Rob Herring <robh+dt@kernel.org>]

On Fri, Jul 12, 2019 at 5:52 PM Saravana Kannan <saravanak@google.com> 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.
>
> Signed-off-by: Saravana Kannan <saravanak@google.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  5 ++
>  drivers/of/platform.c                         | 57 +++++++++++++++++++
>  2 files changed, 62 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 138f6664b2e2..109b4310844f 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3141,6 +3141,11 @@
>                         This can be set from sysctl after boot.
>                         See Documentation/sysctl/vm.txt for details.
>
> +       of_devlink      [KNL] Make device links from common DT bindings. Useful
> +                       for optimizing probe order and making sure resources
> +                       aren't turned off before the consumer devices have
> +                       probed.
> +
>         ohci1394_dma=early      [HW] enable debugging via the ohci1394 driver.
>                         See Documentation/debugging-via-ohci1394.txt for more
>                         info.
> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> index 04ad312fd85b..0930f9f89571 100644
> --- a/drivers/of/platform.c
> +++ b/drivers/of/platform.c
> @@ -509,6 +509,62 @@ int of_platform_default_populate(struct device_node *root,
>  }
>  EXPORT_SYMBOL_GPL(of_platform_default_populate);
>
> +static int of_link_binding(struct device *dev,
> +                          const char *binding, const char *cell)
> +{
> +       struct of_phandle_args sup_args;
> +       struct platform_device *sup_dev;
> +       unsigned int i = 0, links = 0;
> +       u32 dl_flags = DL_FLAG_AUTOPROBE_CONSUMER;
> +
> +       while (!of_parse_phandle_with_args(dev->of_node, binding, cell, i,
> +                                          &sup_args)) {
> +               i++;
> +               sup_dev = of_find_device_by_node(sup_args.np);
> +               of_node_put(sup_args.np);
> +               if (!sup_dev)
> +                       continue;
> +               if (device_link_add(dev, &sup_dev->dev, dl_flags))
> +                       links++;
> +               put_device(&sup_dev->dev);
> +       }
> +       if (links < i)
> +               return -ENODEV;
> +       return 0;
> +}
> +
> +static bool of_devlink;
> +core_param(of_devlink, of_devlink, bool, 0);
> +
> +/*
> + * List of bindings and their cell names (use NULL if no cell names) from which
> + * device links need to be created.
> + */
> +static const char * const link_bindings[] = {
> +       "clocks", "#clock-cells",
> +       "interconnects", "#interconnect-cells",
> +};
> +
> +static int of_link_to_suppliers(struct device *dev)
> +{
> +       unsigned int i = 0;
> +       bool done = true;
> +
> +       if (!of_devlink)
> +               return 0;
> +       if (unlikely(!dev->of_node))
> +               return 0;
> +
> +       for (i = 0; i < ARRAY_SIZE(link_bindings) / 2; i++)
> +               if (of_link_binding(dev, link_bindings[i * 2],
> +                                       link_bindings[i * 2 + 1]))
> +                       done = false;

Given the pending addition of regulators I think this should be
structured a bit differently so that we abstract out the matching and
phandle look-up so there's a clean separation of binding specifics.
It's kind of messy with 2 patterns to parse already and if we added a
3rd? I would iterate over the properties as you do for regulators in
both cases and for each property call a binding specific match
function. The common pattern can of course be a common function. Let
me know if that makes sense. If not I can try to flesh it out some
more.

Rob