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Subject[PATCH v7 0/7] Solve postboot supplier cleanup and optimize probe ordering
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 [1] from
Rafael. Will do that once this patch series is close to being Acked.

[1] -


Saravana Kannan (7):
driver core: Add support for linking devices during device addition
driver core: Add edit_links() callback for drivers
of/platform: Add functional dependency link from DT bindings
driver core: Add sync_state driver/bus callback
of/platform: Pause/resume sync state during init and
of/platform: Create device links for all child-supplier depencencies
of/platform: Don't create device links for default busses

.../admin-guide/kernel-parameters.txt | 5 +
drivers/base/core.c | 168 ++++++++++++++++
drivers/base/dd.c | 29 +++
drivers/of/platform.c | 189 ++++++++++++++++++
include/linux/device.h | 55 +++++
5 files changed, 446 insertions(+)


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