(Mishnah - R. Eliezer): If one did not bring the knife for Milah before Shabbos, one brings it openly on Shabbos;

At a time of danger (Nochri decrees against Milah), one brings it covered, with witnesses (lest people suspect that he carries b'Isur).

R. Eliezer further said that we cut trees to make coals to make a knife [if needed];

R. Akiva says, the general rule is, a Melachah [for the sake of Milah] overrides Shabbos only if it cannot be done before Shabbos.

(Gemara - R. Yochanan) Suggestion: We wanted to say that R. Eliezer argues about everything (all Machshirim, i.e. matters needed for Mitzvos, that they override Shabbos).

Rejection: We learn from R. Eliezer's need to expound about Bikurim [that this is not so];

Just like Bikurim said regarding the Omer overrides Shabbos, also Bikurim said regarding Shtei ha'Lechem overrides Shabbos. This shows that he does not argue about everything. (If he did, he would not need a Drashah for Shtei ha'Lechem!)

Why does R. Eliezer need to expound Bikurim-Bikurim? It is because their primary Dichuy [for the Mitzvah itself] is Shevus, and the Machshirim are Melachah. (However, whenever a Mitzvah itself requires Melachah, even without a Drashah, it and its Machshirim override Shabbos.)

Question: It was taught that Lulav and its Machshirim ovsh [even though there is no verse for this]!

Answer (R. Yonah): This is to bring it from the top of a date tree (it is already detached. Only Machshirim of Shevus, e.g. using a tree, override Shabbos.)

Question: It was taught that Shofar and its Machshirim override Shabbos [even though there is no verse]!

Answer (R. Yonah): This is to bring it from outside the Techum.

Support (Beraisa): We do not scrape [a Shofar from the outside], or scrape [it from the inside] or crush [protrusions that come out from it].