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.
kbuild test robot reported clang error about missing const
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20190904211126.47518-4-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
a3e1d1a7f5
