Here, when talking about 'parent IRQ', I was referring to ERRB or INTB as 'parent IRQ'. My thinking was that, the regmap IRQ instance uses INTB or ERRB as it's parent IRQ, which it splits (demuxes) to separate "child IRQs" for the rest of the PMIC drivers to use. I'd be grateful if better terms were suggested so that readers can stay on same page with me.
After talking with Mark:
we both thought it'd be cleaner to have separate regmap IRQ instances for the ERRB and INTB lines. This makes sense (to me) because a lot of (almost all of?) the regmap IRQ internal data describe the IRQ line related things like registers related to the IRQ line, IRQ line polarity etc. Hence, making single regmap-IRQ instance to support more than one <please, add what is the correct term for INTB / ERRB like line> would require duplicating a plenty of the regmap data. This would make registering an regmap-IRQ entity much more complex and additionally it'd also complicate the internals of the regmap-IRQ. It'd be a bit like trying to fill and drink a six-pack of lemonade at once, instead of going a bottle by bottle :)