GC for Parents - AOS and CP - Information Source
One can petition for parents only if the petitioner is US Citizen. GC holder or LPR cannot apply for parents.
Parents cases of USC are processed differently from the Spouse of USC though they come under the same category.
If parents are already in US, they can go thru the AOS process or CP.
If parents are outside of US, they can go thru the CP only.
For Whom Are You Petitioning?
For AOS or CP, I-130 is the common form.