Add device-links to track functional dependencies between devices after they are created (but before they are probed) by looking at their common DT bindings like clocks, interconnects, etc. Having functional dependencies automatically added before the devices are probed, 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. v1 -> v2: - Drop patch to speed up of_find_device_by_node() - Drop depends-on property and use existing bindings v2 -> v3: - Refactor the code to have driver core initiate the linking of devs - Have driver core link consumers to supplier before it's probed - Add support for drivers to edit the device links before probing TODO: - For the case of consumer child sub-nodes being added by a parent device after late_initcall_sync we might be able to address that by recursively parsing all child nodes and adding all their suppliers as suppliers of the parent node too. The parent probe will add the children before its probe is completed and that will prevent the supplier's sync_state from being executed before the children are probed. But I'll write that part once I see how this series is received. -Saravana Saravana Kannan (4): driver core: Add support for linking devices during device addition of/platform: Add functional dependency link from DT bindings driver core: Add sync_state driver/bus callback driver core: Add edit_links() callback for drivers drivers/base/core.c | 142 +++++++++++++++++++++++++++++++++++++++++ drivers/base/dd.c | 29 +++++++++ drivers/of/Kconfig | 9 +++ drivers/of/platform.c | 61 ++++++++++++++++++ include/linux/device.h | 45 +++++++++++++ 5 files changed, 286 insertions(+) -- 2.22.0.410.gd8fdbe21b5-goog
