whitespace-action

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

nil

Documentation:
Specify which action is taken when a buffer is visited or written.

It's a list containing some or all of the following values:

nil no action is taken.

cleanup cleanup any bogus whitespace always when local
whitespace is turned on.
See `whitespace-cleanup' and
`whitespace-cleanup-region'.

report-on-bogus report if there is any bogus whitespace always
when local whitespace is turned on.

auto-cleanup cleanup any bogus whitespace when buffer is
written.
See `whitespace-cleanup' and
`whitespace-cleanup-region'.

abort-on-bogus abort if there is any bogus whitespace and the
buffer is written.

warn-if-read-only give a warning if `cleanup' or `auto-cleanup'
is included in `whitespace-action' and the
buffer is read-only.

Any other value is treated as nil.

You can customize this variable.