Use a blender or processor to grind the ginger, 1 teaspoon cumin and green chilli.
In a medium saucepan, add water, pineapple, tomato and salt. Place over a low heat. When the pineapple and water have come to boil, add the ginger mixture. Raise and lower the heat to bring to the boil several times. Add the remaining water, and bring to the boil again. Add coriander, and remove from heat. Mix in lime juice.
In a small pan, heat oil over medium heat, and add the remaining cumin seeds. Fry until golden, and add to the soup as a garnish.
Roll limes against a hard surface with the palm of your hand to make them soft. This helps get the juice out.