[Bug 790940] New: v4l2src not falling back to another io-method if driver does not support dmabuf

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=790940

            Bug ID: 790940
           Summary: v4l2src not falling back to another io-method if
                    driver does not support dmabuf
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-good
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: florent.thiery at ubicast.eu
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Created attachment 364562
  --> https://bugzilla.gnome.org/attachment.cgi?id=364562&action=edit
error log

Hello,

It looks like latest master seems to auto-select dmabuf event if the device
doesn't support it, without any falling back to a "safe" method (e.g. mmap).

0:00:00.037997607   441 0x5567a0579450 DEBUG         v4l2bufferpool
gstv4l2bufferpool.c:743:gst_v4l2_buffer_pool_start:<v4l2src0:pool:src>
activating pool
0:00:00.038003304   441 0x5567a0579450 DEBUG         v4l2bufferpool
gstv4l2bufferpool.c:773:gst_v4l2_buffer_pool_start:<v4l2src0:pool:src>
requesting 4 MMAP buffers
0:00:00.038012154   441 0x5567a0579450 DEBUG          v4l2allocator
gstv4l2allocator.c:692:gst_v4l2_allocator_start:<v4l2src0:pool:src:allocator>
allocated 4 mmap buffers out of 4 requested
0:00:00.038027996   441 0x5567a0579450 WARN          v4l2bufferpool
gstv4l2bufferpool.c:790:gst_v4l2_buffer_pool_start:<v4l2src0:pool:src>
Uncertain or not enough buffers, enabling copy threshold
0:00:00.038040788   441 0x5567a0579450 ERROR          v4l2allocator
gstv4l2allocator.c:906:gst_v4l2_allocator_alloc_dmabuf:<v4l2src0:pool:src:allocator>
Failed to export DMABUF: Inappropriate ioctl for device
0:00:00.038045683   441 0x5567a0579450 ERROR         v4l2bufferpool
gstv4l2bufferpool.c:479:gst_v4l2_buffer_pool_alloc_buffer:<v4l2src0:pool:src>
failed to allocate buffer
0:00:00.038050221   441 0x5567a0579450 ERROR         v4l2bufferpool
gstv4l2bufferpool.c:898:gst_v4l2_buffer_pool_start:<v4l2src0:pool:src> allocate
failed
0:00:00.038053892   441 0x5567a0579450 ERROR             bufferpool
gstbufferpool.c:564:gst_buffer_pool_set_active:<v4l2src0:pool:src> start failed
0:00:00.038059392   441 0x5567a0579450 WARN                 v4l2src
gstv4l2src.c:620:gst_v4l2src_decide_allocation:<v4l2src0> error: Failed to
allocate required memory.
0:00:00.038063077   441 0x5567a0579450 WARN                 v4l2src
gstv4l2src.c:620:gst_v4l2src_decide_allocation:<v4l2src0> error: Buffer pool
activation failed
ERROR: from element /GstPipeline:pipeline0/GstV4l2Src:v4l2src0: Failed to
allocate required memory.

I can work around this by forcing io-mode=2 for instance, but why is 0 (auto)
selecting dmabuf in the first place ?

Attached is the output of:
$ GST_DEBUG=v4l*:5 gst-launch-1.0 v4l2src device=/dev/video0 num-buffers=100 !
videoconvert ! videoscale ! "video/x-raw, format=(string)NV12, width=(int)1920,
height=(int)1200, framerate=(fraction)30, colorimetry=(string)bt709" ! fakesink

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