Preheat oven to 350º. Put rhubarb and ginger into a medium bowl and toss together, mixing thoroughly. Transfer to a large baking dish. Sprinkle 2 cups of the sugar over rhubarb mixture and set aside.
Whisk flour, salt, and the remaining 1⁄2 cup sugar together in a medium bowl. Using a pastry cutter or 2 table knives, work butter into flour mixture until it resembles coarse meal flecked with pea-size pieces of butter. Scatter topping evenly over rhubarb mixture.
Transfer dish to oven and bake until topping is golden brown and rhubarb is soft, about 1 hour 15 minutes. Set aside to cool on a wire rack at least 10 minutes before serving warm, or allow to cool to room temperature.