R. Porat (or: don Bendit, הר"י פורת) was named Yosef b. Moshe [of Troyes]. "Porat" or "Bechor Shor" were recognized sobriquets for those named Yosef (taken from Birchot Yaakov, Gen. 49:22 & Birchot Moshe, Deut. 33:13). Contrary to Zacuto et al., Solomon Schechter (here) dismisses the notion that Porat was a son of Rashbam and only considers being a grandson a possibility. E.E. Urbach (Ba'alei Tosafos, Jer. 1968, pg. 99). rejects both views as being without basis and maintains Porat was not a son nor a grandson.
Incidentally, Urbach demonstrates (ibid. pg. 478) that the Tosafos on Yoma were edited by R. Meir of Rothenburg and actually cites the above Tosafos as an example proving his position. See also Urbach (loc. cit.) for a description of Porat's literary production.