Date:   Mon, 1 Jul 2019 19:31:47 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        David Collins <collinsd@...eaurora.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v3 2/4] of/platform: Add functional dependency link from
 DT bindings

On Mon, Jul 1, 2019 at 6:48 PM Saravana Kannan <saravanak@...gle.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@...gle.com>
> ---
>  drivers/of/Kconfig    |  9 ++++++++
>  drivers/of/platform.c | 52 +++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 61 insertions(+)
>
> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> index 37c2ccbefecd..7c7fa7394b4c 100644
> --- a/drivers/of/Kconfig
> +++ b/drivers/of/Kconfig
> @@ -103,4 +103,13 @@ config OF_OVERLAY
>  config OF_NUMA
>         bool
>
> +config OF_DEVLINKS

I'd prefer this not be a config option. After all, we want one kernel
build that works for all platforms.

A kernel command line option to disable might be useful for debugging.

> +       bool "Device links from DT bindings"
> +       help
> +         Common DT bindings like clocks, interconnects, etc represent a
> +         consumer device's dependency on suppliers devices. This option
> +         creates device links from these common bindings so that consumers are
> +         probed only after all their suppliers are active and suppliers can
> +         tell when all their consumers are active.
> +
>  endif # OF
> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> index 04ad312fd85b..a53717168aca 100644
> --- a/drivers/of/platform.c
> +++ b/drivers/of/platform.c
> @@ -61,6 +61,57 @@ struct platform_device *of_find_device_by_node(struct device_node *np)
>  EXPORT_SYMBOL(of_find_device_by_node);
>
>  #ifdef CONFIG_OF_ADDRESS
> +static int of_link_binding(struct device *dev, char *binding, char *cell)

Under CONFIG_OF_ADDRESS seems like a strange location.

> +{
> +       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);
> +               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;
> +}
> +
> +/*
> + * List of bindings and their cell names (use NULL if no cell names) from which
> + * device links need to be created.
> + */
> +static char *link_bindings[] = {

const

> +#ifdef CONFIG_OF_DEVLINKS
> +       "clocks", "#clock-cells",
> +       "interconnects", "#interconnect-cells",

Planning to add others?

> +#endif
> +};
> +
> +static int of_link_to_suppliers(struct device *dev)
> +{
> +       unsigned int i = 0;
> +       bool done = true;
> +
> +       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;
> +
> +       if (!done)
> +               return -ENODEV;
> +       return 0;
> +}
> +
>  /*
>   * The following routines scan a subtree and registers a device for
>   * each applicable node.
> @@ -524,6 +575,7 @@ static int __init of_platform_default_populate_init(void)
>         if (!of_have_populated_dt())
>                 return -ENODEV;
>
> +       platform_bus_type.add_links = of_link_to_suppliers;
>         /*
>          * Handle certain compatibles explicitly, since we don't want to create
>          * platform_devices for every node in /reserved-memory with a
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

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