Pinctrl

Many embedded Linux device bugs look like register-access bugs: probe runs, register mapping succeeds, but reads return invalid values; interrupts never arrive; an I2C device randomly NACKs; the first access after resume fails.

The problem is not always in register access.

Whether hardware works often depends on more basic resources first: pins must be muxed correctly, clocks must be enabled, power must be stable, reset must be released, and GPIO polarity must be correct.