Rashi opens his commentary to the Chumash with the words Amar Rabbi Yitzchak, "Rabbi Yitzchak said." However I am unsure the identity of this Rabbi Yitzchak. I have been unable to find a source for this question in the Talmud, Midrash, etc. Was this Rabbi Yitzchak actually Rashi's father?
(I am aware of what Siftei Chachamim says regarding this, and although they dismiss the idea that Yitzchak, Rashi's father, was an unlearned man, they do not necessarily rule out that the Rabbi Yitzchak mentioned here was Rashi's father.)
Who is the Rabbi Yitzchak mentioned here?