Re: [PATCH v7 0/7] Solve postboot supplier cleanup and optimize probe ordering
From: Greg Kroah-Hartman
Date: Thu Jul 25 2019 - 14:06:43 EST
On Tue, Jul 23, 2019 at 05:10:53PM -0700, Saravana Kannan wrote:
> 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
> - 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
> v3 -> v4:
> - Tested edit_links() on system with cyclic dependency. Works.
> - Added some checks to make sure device link isn't attempted from
> parent device node to child device node.
> - Added way to pause/resume sync_state callbacks across
> - Recursively parse DT node to create device links from parent to
> suppliers of parent and all child nodes.
> v4 -> v5:
> - Fixed copy-pasta bugs with linked list handling
> - Walk up the phandle reference till I find an actual device (needed
> for regulators to work)
> - Added support for linking devices from regulator DT bindings
> - Tested the whole series again to make sure cyclic dependencies are
> broken with edit_links() and regulator links are created properly.
> v5 -> v6:
> - Split, squashed and reordered some of the patches.
> - Refactored the device linking code to follow the same code pattern for
> any property.
> v6 -> v7:
> - No functional changes.
> - Renamed i to index
> - Added comment to clarify not having to check property name for every
> - Added "matched" variable to clarify code. No functional change.
> - Added comments to include/linux/device.h for add_links()
> I've also not updated this patch series to handle the new patch  from
> Rafael. Will do that once this patch series is close to being Acked.
>  - https://lore.kernel.org/lkml/3121545.4lOhFoIcdQ@kreacher/
This looks sane to me. Anyone have any objections for me queueing this
up for my tree to get into linux-next now?