N2
だけは
To do all that one can, As much or many ~ as one could
飲み放題だったから、飲めるだけは飲んだ。
It was all you could drink, so I drank as much as I could.
Explanation
「だけは」 has 2 uses
1All you can
「verb＋だけは＋the same verb」: you did as much as you could, and the rest is out of your hands: 「やるだけはやった」 (I did all I could), 「食べられるだけは食べた」 (I ate as much as I could).
食べられるだけは食べた。食べ放題だからさ。
I ate all that I could. It was an all you can eat after all.
飲み放題だったから、飲めるだけは飲んだ。
It was all you could drink, so I drank as much as I could.
2At least this
Whatever else, this one thing was done or must be done; 「〜するだけはしたが」 often means 'I did do it, but…': 「買うだけは買ったが、使っていない」 (I did buy it, but I don't use it).
ピアノのレッスンには通うだけは通っているけど、ピアノにはあまり興味がない。
I do go to piano lessons, but I'm not very interested in the piano.
毎日漢字の勉強をするだけはしているが、上達している感じがしない。
I do study kanji every day, but I don't feel I'm getting better.