Spread rhubarb into a 1 1/2-quart casserole dish to cover the bottom completely. Sprinkle white sugar evenly over the rhubarb.
Mix flour, brown sugar, ginger, and cinnamon together in a large bowl. Blend butter into the sugar mixture with a pastry cutter until the mixture forms crumbs the size of a small pea; spread over the rhubarb.
Bake in preheated oven until the top is lightly browned, about 30 minutes. Cool crisp before serving.
