v4l2src (1.8.1) why requires downstream pool in userptr mode

[Jake Zhang]

I am tring to use userptr mode of v4l2src on my ARM board with below
pipeline but v4l2src is complaining no downstream pool is available. I am
wondering why v4l2src is not using the v4l2bufferpool but expect a
bufferpool from downstream.

------------------------------------------
gst-launch-1.0 v4l2src num-buffers=1 io-mode=3 device=/dev/video0
do-timestamp=true ! 'video/x-raw, width=1280, height=720, framerate=30/1,
format=YUY2' ! videoconvert !  fakesink
------------------------------------------
======= more detailed error log ===============
0:00:00.651854958  4427      0x2325370 INFO                    v4l2
gstv4l2object.c:2776:gst_v4l2_object_setup_pool:<v4l2src0> accessing
buffers via mode 3
0:00:00.652293059  4427      0x2325370 INFO          v4l2bufferpool
gstv4l2bufferpool.c:551:gst_v4l2_buffer_pool_set_config:<v4l2src0:pool:src>
increasing minimum buffers to 2
0:00:00.652366954  4427      0x2325370 INFO          v4l2bufferpool
gstv4l2bufferpool.c:564:gst_v4l2_buffer_pool_set_config:<v4l2src0:pool:src>
reducing maximum buffers to 32
0:00:00.652373350  4427      0x2325370 INFO          v4l2bufferpool
gstv4l2bufferpool.c:575:gst_v4l2_buffer_pool_set_config:<v4l2src0:pool:src>
can't allocate, setting maximum to minimum
0:00:00.653270760  4427      0x2325370 WARN                    v4l2
gstv4l2object.c:3831:gst_v4l2_object_decide_allocation:<v4l2src0> decide
allocation
0:00:00.653294184  4427      0x2325370 WARN                    v4l2
gstv4l2object.c:3857:gst_v4l2_object_decide_allocation:<v4l2src0>
allocation: size:0 min:0 max:0 pool:(NULL)
0:00:00.653357210  4427      0x2325370 WARN                    v4l2
gstv4l2object.c:4087:gst_v4l2_object_decide_allocation:<v4l2src0> error: No
downstream pool to import from.
0:00:00.653365993  4427      0x2325370 WARN                    v4l2
gstv4l2object.c:4087:gst_v4l2_object_decide_allocation:<v4l2src0> error:
When importing DMABUF or USERPTR, we need a pool to import from
ERROR: from element /GstPipeline:pipeline0/GstV4l2Src:v4l2src0: No
downstream pool to import from.
=============================================


To make v4l2src happy, I add a 'videomedian' element which has a bufferpool
and now I can see v4l2src is using downstream buffer pool but it can not
request the buffer. If I swtich to DMABUF mode and hack the v4l2src to
force it use downstream pool, it will be no issue of requesting buffer.
I must have missed something about how to use userptr mode in v4l2src,
could anyone provide some input here? Thanks
------------------------------
gst-launch-1.0 v4l2src num-buffers=1 io-mode=3 device=/dev/video0
do-timestamp=true ! 'video/x-raw, width=1280, height=720, framerate=30/1' !
videoconvert !  videomedian ! fakesink
------------------------
======= more detailed error log by adding videomedian ===============
0:00:00.677143473  5218       0xa88230 ERROR          v4l2allocator
gstv4l2allocator.c:738:gst_v4l2_allocator_start:<v4l2src0:pool:src:allocator>
error requesting 2 buffers: Invalid argument
0:00:00.677153347  5218       0xa88230 ERROR         v4l2bufferpool
gstv4l2bufferpool.c:848:gst_v4l2_buffer_pool_start:<v4l2src0:pool:src> we
received 0 buffer from device '/dev/video0', we want at least 2
0:00:00.677271601  5218       0xa88230 WARN                 v4l2src
gstv4l2src.c:511:gst_v4l2src_decide_allocation:<v4l2src0> error: Failed to
allocate required memory.
0:00:00.677278901  5218       0xa88230 WARN                 v4l2src
gstv4l2src.c:511:gst_v4l2src_decide_allocation:<v4l2src0> error: Buffer
pool activation failed
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