whitespace-global-modes

whitespace-global-modes is a variable defined in `whitespace.el'.
Its value is

t

Documentation:
Modes for which global `whitespace-mode' is automagically turned on.

Global `whitespace-mode' is controlled by the command
`global-whitespace-mode'.

If nil, means no modes have `whitespace-mode' automatically
turned on.

If t, all modes that support `whitespace-mode' have it
automatically turned on.

Else it should be a list of `major-mode' symbol names for which
`whitespace-mode' should be automatically turned on. The sense
of the list is negated if it begins with `not'. For example:

(c-mode c++-mode)

means that `whitespace-mode' is turned on for buffers in C and
C++ modes only.

You can customize this variable.