The reason of removal, "replaced by allowdac and no dac
combination" is incorrect. There is no way to do the same thing
with "allowdac" and "nodac" combination.

The usedac option enables us to stop via_no_dac() setting
forbid_dac to 1. That is, someone who uses VIA bridges can use
DAC with this option even if some of VIA bridges seem to be
broken about DAC.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: WANG Cong <amwang@redhat.com>
Cc: gcosta@redhat.com
LKML-Reference: <20091214104423X.fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>