Merge series "regulator_sync_state() support" from Saravana Kannan <saravanak@google...

Merge series "regulator_sync_state() support" from Saravana Kannan <saravanak@google.com>:

Consider the following example:
- regulator-X is provided by device-X.
- regulator-X is a supplier to device-A, device-B and device-C.
- device-A is off/inactive from boot.
- device-B and device-C are left on/active by the bootloader
- regulator-X is left on boot by the bootloader at 2000 mV to supply
  device-B and device-C.

Example boot sequence 1:
1. device-X is probed successfully.
2. device-A is probed by driver-A
   a. driver-A gets regulator-X
   b. driver-A votes on regulator-X
   c. driver-A initializes device-A
   d. driver-A votes off regulator-X
   e. regulator-X is turned off.
3. System crashes or device-B and device-C become unreliable because
   regulator-X was turned off without following the proper quiescing
   steps for device-B and device-C.

Example boot sequence 2:
1. device-X is probed successfully.
2. device-B is probed by driver-B
   a. driver-B gets regulator-X
   b. driver-B votes on regulator-X
   c. driver-B lowers device-B performance point.
   d. driver-B lowers voltage vote to 1000 mV.
   e. regulator-X voltage is lowered to 1000 mV.
3. System crashes or device-C becomes unreliable because regulator-X
   voltage was lowered to 1000 mV when device-C still needed it at 2000 mV

This patch series makes sure these examples are handled correctly and
system crash or device instability is avoided and the system remains
usable.

More details provided in the commit texts.

v2->v3:
Patch 2/4 - No functional change. Simple refactor.
Patch 3/4
- Was Patch 2/2 in v2.
- Rewrote commit text to hopefully address all previous points.
- Renamed variable/functions. Hope it's clearer.
- Added more comments.
- Added logging
- Fixed timeout functionality.
- Handle exclusive consumers properly
- Handle coupled regulators properly
Patch 4/4 - Prevents voltage from going too low during boot.

v1->v2:
Patch 1/2
- New patch
Patch 2/2
- This was the only patch in v1
- Made the late_initcall_sync timeout a commandline param
- If timeout is set, we also give up waiting for all consumers after
  the timeout expires.
- Made every regulator driver add sync_state() support

Saravana Kannan (4):
  driver core: Add dev_set_drv_sync_state()
  regulator: core: Add destroy_regulator()
  regulator: core: Add basic enable/disable support for sync_state()
    callbacks
  regulator: core: Add voltage support for sync_state() callbacks

 drivers/regulator/core.c         | 200 ++++++++++++++++++++++++++++---
 include/linux/device.h           |  12 ++
 include/linux/regulator/driver.h |   2 +
 3 files changed, 198 insertions(+), 16 deletions(-)

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2.28.0.rc0.105.gf9edc3c819-goog