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The vb2 read support requests 1 buffer, leaving it to the driver
to increase this number to something that works.

Unfortunately, drivers do not deal with this reliably, and in fact
this caused problems for the bttv driver and reading from /dev/vbiX,
causing every other VBI frame to be all 0.

Instead, request as the number of buffers whatever is the maximum of
2 and q->min_buffers_needed+1.

In order to start streaming you need at least q->min_buffers_needed
queued buffers, so add 1 buffer for processing. And if that field
is 0, then choose 2 (again, one buffer is being filled while the
other one is being processed).

This certainly makes more sense than requesting just 1 buffer, and
the VBI bttv support is now working again.

It turns out that the old videobuf1 behavior of bttv was to allocate
8 (video) and 4 (vbi) buffers when used with read(). After the vb2
conversion that changed to 2 for both. With this patch it is 3, which
is really all you need.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: b7ec3212

 ("media: bttv: convert to vb2")
Tested-by: Dr. David Alan Gilbert <dave@treblig.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>