So it takes donation not from the miners directly, but from a donation set aside from the total supply.

Does the vote come only from the miners who find the block ?

It looks to me from looking only at that one block of code (so this may be wrong), that donations are computed from a fixed amount of the total supply (1% I guess) less the amount of previous donations. So the effect is that as long as one miner votes for the donation, the entire donation amount (1% of the total supply) will eventually be donated (over a similar timescale to the overall emission curve). Votes no don't reduce the total donation, they reduce the current donation but increase future donations, in effect slowing down donations but not ultimately changing the total.

But I haven't studied this that carefully so I may be wrong.

Yes, and not really. I think if you solo mine, then you can turn off the 1% donation, but if you mine at a pool, then I think they mostly have it turned on. It's really not clear, and I've asked for more information previously, and was told to read the source code. It would be nice if there was more than a vague sentence or two about miners voting on dev bounty for non-c++ programmers.