Manava Dharmashastra 3.212

3.212

yathā jīvatpitā bhojyate tadvatpitāmaho'pi| anujñāṃ pitāmahāt prāpya samācaretsvavayam| parato dvayordadyāt, prapitāmahāya ekasmai eva vā| eṣa ‘kāmaṃ' ‘svayam' ityanayorarthaḥ |

Manu has declared that either the grandfather may eat at that Śrāddha, or the man himself may, according to his will, perform it, after being permitted by him.—(222)

Medhātithi’s commentary (manubhāṣya):
Just as the living father is fed, so is the grandfather also.

Having sought permission from the grandfather, the man himself may perform the śrāddha; i.e., he should make the offering to the two remoter ancestors, or to the great-grandfather only. This is what is implied by the terms ‘may’ and ‘according to his will.’—(222)