Let $q$ be a power of 2.  Let $P$ be the set of  polynomials in 
$F_q [x]$ of degree d or less.
Let $\mathbb{Z}$ be the ring of integers. For any $f \in P$, let $\psi(f)$ be the
number of distinct roots of $f$ in $F_q$. Note that $\psi(0) = q$.

For any map  $A$ from $P$ to $\mathbb{Z}$, one can  compute the summation
  $$\sum_{f, g \in P} A(f)A(g) \psi (f+g).$$
My question is:
what is the minimum positive value of the summation?

For $d=0,1$, the minimum is $q$. What happens if $d$ is bigger?
I am especially interested in the case when d=q/2-1.

Thanks a lot,

Qi