The I2C-bus to the XPower AXP288 is shared between the Linux kernel and
the SoCs P-Unit. The P-Unit has a semaphore which the kernel must "lock"
before it may use the bus and while the kernel holds the semaphore the CPU
and GPU power-states must not be changed otherwise the system will freeze.

This is a complex process, which is quite expensive. This is all done by
iosf_mbi_block_punit_i2c_access(). To ensure that no unguarded I2C-bus
accesses happen, iosf_mbi_block_punit_i2c_access() gets called by the
I2C-bus-driver for every I2C transfer. Because this is so expensive it
is allowed to call iosf_mbi_block_punit_i2c_access() in a nested
fashion, so that higher-level code which does multiple I2C-transfers can
call it once for a group of transfers, turning the calls done by the
I2C-bus-driver into no-ops.

Userspace power-supply API users typically will read all provided
properties in one go, refreshing the last read values when
power_supply_changed() is called by the driver and/or periodically
(e.g. every 2 minutes).

The reading of all properties in one go causes the P-Unit semaphore
to quickly be taken and released multiple times in a row. Certain
PMIC registers like AXP20X_FG_RES are even used in multiple properties
so they get read multiple times, leading to a P-Unit take + release
each time the register is read.

As already mentioned the taking of the P-Unit semaphore is a quite
expensive operation and it has also been reported that the
"hammering" of the P-Unit semaphore done by the axp288_fuel_gauge
driver can even cause stability issues with the system as a whole.

Switch over to a scheme where the axp288_fuel_gauge driver keeps
a local copy of all the registers which it uses for properties
and make it only refresh its copy of the registers if the values
are older then 1 minute; or when a fuel-gauge interrupt has
triggered since the last read.

This not only reduces the amount of reads, it also makes the code
do all the reads in one go, rather then reading specific registers
based on which property is being queried. This allows calling
iosf_mbi_block_punit_i2c_access() once before doing all the reads,
so that we now only take the P-Unit semaphore once per update.
Tested-by: Andrejus Basovas <cpp@gcc.lt>
Signed-off-by: Andrejus Basovas <cpp@gcc.lt>
Co-developed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>