def read(self, indexes=None, out=None, window=None, masked=False,
        boundless=False):
"""Read raster bands as a multidimensional array
    Parameters
    ----------
    window : a pair (tuple) of pairs of ints, optional
        The optional `window` argument is a 2 item tuple. The first
        item is a tuple containing the indexes of the rows at which
        the window starts and stops and the second is a tuple
        containing the indexes of the columns at which the window
        starts and stops. For example, ((0, 2), (0, 2)) defines
        a 2x2 window at the upper left of the raster dataset.
"""

def block_windows(self, bidx=0):
    """Returns an iterator over a band's block windows and their
    indexes.
    [....]
    The primary use of this function is to obtain windows to pass to
    read_band() for highly efficient access to raster block data.
    """

with rasterio.open("your/data/geo.tif") as src:
     for block_index, window in src.block_windows(1):
         block_array = src.read(window=window)
         result_block = some_calculation(block_array)