On Mon, Jul 1, 2019 at 6:48 PM 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.
>
> Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>
> ---
> 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
>
