Hi Nicolas,
On 05/25/2018 06:21 PM, Nicolas Dufresne wrote:
Le vendredi 25 mai 2018 à 21:14 -0400, Nicolas Dufresne a écrit :
Le vendredi 25 mai 2018 à 17:19 -0700, Steve Longerbeam a écrit :
On 05/25/2018 05:10 PM, Nicolas Dufresne wrote:
(in text this time, sorry)
Le vendredi 25 mai 2018 à 16:53 -0700, Steve Longerbeam a écrit :
Add a macro that returns true if the given field type is
'sequential',
that is, the data is transmitted, or exists in memory, as all top
field
lines followed by all bottom field lines, or vice-versa.
Signed-off-by: Steve Longerbeam <steve_longerb...@mentor.com>
---
include/uapi/linux/videodev2.h | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/include/uapi/linux/videodev2.h
b/include/uapi/linux/videodev2.h
index 600877b..408ee96 100644
--- a/include/uapi/linux/videodev2.h
+++ b/include/uapi/linux/videodev2.h
@@ -126,6 +126,10 @@ enum v4l2_field {
(field) == V4L2_FIELD_INTERLACED_BT ||\
(field) == V4L2_FIELD_SEQ_TB ||\
(field) == V4L2_FIELD_SEQ_BT)
+#define V4L2_FIELD_IS_SEQUENTIAL(field) \
+ ((field) == V4L2_FIELD_SEQ_TB ||\
+ (field) == V4L2_FIELD_SEQ_BT ||\
+ (field) == V4L2_FIELD_ALTERNATE)
No, alternate has no place here, in alternate mode each buffers have
only one field.
Then I misunderstand what is meant by "alternate". The name implies
to me that a source sends top or bottom field alternately, or top/bottom
fields exist in memory buffers alternately, but with no information about
which field came first. In other words, "alternate" is either seq-tb or
seq-bt,
without any info about field order.
If it is just one field in a memory buffer, why isn't it called
V4L2_FIELD_TOP_OR_BOTTOM, e.g. we don't know which?
I don't see why this could be better then ALTERNATE, were buffers are
only top or bottom fields alternatively. And even if there was another
possible name, this is public API.
V4L2_FIELD_ALTERNATE is a mode, that will only be used with
v4l2_pix_format or v4l2_pix_format_mplane. I should never bet set on
the v4l2_buffer.field, instead the driver indicates the parity of the
field by setting V42_FIELD_TOP/BOTTOM on the v4l2_buffer returned by
DQBUF. This is a very different mode of operation compared to
sequential, hence why I believe it is wrong to make it part of the new
helper. So far, it's the only field value that has this asymmetric
usage and meaning.
I should have put some references. The explanation of the modes, with a
temporal representation of the fields. Small note, in ALTERNATE mode
bottom and top fields will likely not share the same timestamp, it is a
mode used to achieve lower latency.
https://linuxtv.org/downloads/v4l-dvb-apis/uapi/v4l/field-order.html#c.v4l2_field
And in this section, you'll see a paragraph that explain the field
values when running in ALTERNATE mode.
https://linuxtv.org/downloads/v4l-dvb-apis/uapi/v4l/buffer.html#c.v4l2_buffer
Ok, thanks for clarifying. I suppose it makes sense that this mode
should not
be seen as sequential, since it represents a mode where each userspace
buffer
is a single, self-contained field. I'll remove ALTERNATE from the helper
macro.
Steve
