Currently vivid supports the Request API, but it also sets
min_buffers_needed in the vb2 queue. But the combination of
support_requests and min_buffers_needed is not allowed due to
the fact that vb2_core_qbuf() isn't supposed to fail when called
from the request framework. And if min_buffers_needed > 0, then
is can call start_streaming() which definitely can fail.

With the new module option you can control if requests are not
allowed (min_buffers_needed is 2 in that case), optionally allowed
or are required. In the latter two cases min_buffers_needed is set
to 0.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>