What is the proper way to read RTC battery voltage

[Lei YU]

This email is to ask about a proper way to read RTC battery voltage.

Background:
* Usually RTC battery voltage could be read from an ADC channel.
* Usually there is a GPIO to enable/disable the connection between battery
   and ADC, e.g.
   * When GPIO is pulled low, the channel is disconnected and reading is 0.
   * When GPIO is pulled high, the channel is connected and the reading is OK.
* So to read the voltage, the GPIO shall be pulled high.
* When the GPIO is pulled high, it drains the battery.

The problem: How to properly read the voltage, without affecting the battery
life?

1. If the GPIO is simply hogged high in device tree, the ADC channel always get
   the correct reading, but it affects the battery life.
2. We could leave the GPIO for user space to control, e.g. let phosphor-hwmon
   to control the GPIO while reading a sensor, it resolves the problem.
   This is done by Google and the patch for phosphor-hwmon is at
   (https://gerrit.openbmc-project.xyz/#/c/openbmc/phosphor-hwmon/+/11697/)
3. We could use gpio-mux and io-channel-mux driver, that let kernel to
   automatically pull the GPIO while reading the sensor. (See
   https://github.com/mine260309/linux/commit/8e60647389e5400a85c705685ca2f6b875856c99
   for example).
   However, it requires some delay between GPIO pulled high and reading the ADC
   channel, if the delay is too small, the ADC channel reading becomes
   incorrect, e.g. it gets 0 or a lower value than expected.

In the above 3 ways:
1 is not acceptable;
2 is OK, but it is not as clean as handle the GPIO in kernel;
3 is more preferred, but it requires a delay in kernel. To upstream this
   change, the delay shall be put in a proper place, e.g. in iio-mux driver,
   or in adc driver?
   I do not think either place is proper for the delay...

So my question is, which is the proper way to handle the case, or is there any
other way?

Thanks!