Do you have to apply the Drunkard's Luck feature before the roll?

Question

The Way of the Drunken Master grants the ability, at 11th level, to cancel an attack/check/save disadvantage by using 2 ki points.

My question is : do you have to use this feature before the roll, or can you use it after (but before the results) ?

And if you can use the feature after the roll, do you decide which of the two dice you keep ?

I believe that you indeed must use the feature before the roll, or else, you could essentially turn a disadvantage into an advantage (keeping the best of two). But I'm not sure.

Answer 1

Before the roll only

When you make an ability check, an attack roll, or a saving throw and have disadvantage on the roll, you can spend 2 ki points to cancel the disadvantage for that roll. (XGtE)

The fact that it says "when you make" which means it has to be at the time you roll the dice and not after.

Additionally, if a feature allows you use it after rolling, but before (or even after) seeing the results it always says so.

For example, the Bard's Cutting Word ability start with very similar wording, but specifies later in the ability that it can be used later than the default (at the same time the roll is made):

When a creature that you can see within 60 feet of you makes an attack roll, an ability check, or a damage roll, you can use your reaction to expend one of your uses of Bardic Inspiration, rolling a Bardic Inspiration die and subtracting the number rolled from the creature’s roll. You can choose to use this feature after the creature makes its roll, but before the DM determines whether the attack roll or ability check succeeds or fails, or before the creature deals its damage. (PHB)

If an ability does not say that it does something and there is no general rule saying that it should do that thing, then it cannot do that thing.