The Mechaber (O.C. 139) writes that in places where the person called for an aliya leins the parsha from the Torah, he must prepare the parsha in advance or not go up for his aliya. The Rama notes that our custom is to have a designated ba’al koreh who is responsible for learning the parsha and leining all the aliyos. However, the Mishna Berura (s”k 3) writes that it is still “mitzvah min hamuvchar” for a person who gets an aliya to have prepared the parsha, and reviewing the parsha by doing shnayim mikra in advance is sufficient.
Another reason to try to finish shnayim mikra by Shabbos morning!