The gemara tells about Rav, Shmuel, and R' Asi all coming to a שבוע הבן or a ישוע הבן. Rashi comments that the שבוע הבן refers to a bris milah, whereas the ישוע הבן refers to a "party that was done for pidyon haben". The maharsha (agados) points out that from rashi we find a source for making a party for pidyon haben, but is not a source for making a party for a bris milah. The maharsha seems to understand that the requirement to make a party for pidyon haben is better sourced than the requirement to make one for bris milah.
However, the Maharshal (yam shel shlomo 37), in his famous discussion about seudas mitzvah asks a question from the gemara in chulin 95b was not to eat at any "סעודת הרשות". How then can Rav join in the seudah? The maharshal explains the question - בשלמא מילה, הסעודה גופא היא שמחת מצוה כמו סעודות חתן שנאמר "שש אנכי על אמרתך" אבל פדיון הבן ליכא סעודת מצוה להדיא. The maharshal clearly understands that whether it was milah or pidyon haben, Rav was joining and eating at the party. He assumes that the requirement to have a seudah for milah is more basic than having one at pidyon haben. Since the seudah of pidyon haben is only a "reshus", how could Rav join and eat ?(Rashi in chulin seems to avoid the question by saying that the seudah was a בת ת"ח לע"ה which is worse than just a reshus).
From this the maharshal is mechadesh a major yesod that any seudah whos purpose is to give praise to Hashem and either publicize a mitzvah (such as pidyon haben) or a miracle (such as the birth of a child which is Rabbeinu Tam's explanation of ישוע הבן i.e. a shalom zachor) qualifies as a seduas mitzvah.
The source for a siyum on a masechta of gemara qualifying as a seudas mitzvah, the maharshal says is from a gemara in shabbos 118b that Abbaye would make a party when he would see a talmid chacham finish a masechet (the maharshal then launches into a suggestion to make the bracha of השמחה במעונו at a siyum, which he retracted from after he felt that it was the reason that a siyum he once attended was totally ruined). At the end of the perek, the maharshal continues to show from this gemara in shabbos that even those who aren't actually finishing the masechet should celebrate with the one completing the masechet, just as we find that abbaye would make the seudah for his students even when he didn't actually learn it with them. The Maharshal also points out that the gemara in Ta'anis 30b cites that one of the reasons for establishing a yom tov on the 15 of Av was because it was the day that they completed the mitzvah of cutting the wood for the mizbeiach. Just as there is an inyan to make a seudah and yom tov upon the completion fo a mitzvah, so too with the completion of a masechet because "השלמת הספר אין לך מצוה גדולה מזו". As suprising as it may seem, the seudah at the siyum masechet seems to be better sourced as a seudas mitzvah, more than bris milah (which the maharsha questions) and pidyon haben (which the maharshal initially questioned).