UDL's fbdev doesn't work for user-space apps

[Alexey Brodkin <Alexey.Brodkin@synopsys.com>]

Hello,

I'm trying to use DisplayLink USB2.0-to-HDMI adapter as the one and only
video output and I want to get Xserver working on top of that.

I'm not very familiar with all the parts of Linux GPU/video stack
(especially its user-space counterpart) so my assumptions might be wrong
in that case please correct me.

My first [probably incorrect] assumption is Xserver requires fbdev (/dev/fbX)
and it cannot use DRI video card natively. Is that correct?

So to get /ded/fb0 with UDL I just enabled CONFIG_DRM_UDL & CONFIG_DRM_FBDEV_EMULATION.
That gave me boot console on HDMI screen and I was full of expectations.
But when I tried to use /dev/fb0 from whatever user-space app nothing got
displayed on the screen... as well as no error messages appeared.

After eyeballing at UDL code (especially in comparison with QXC which uses deferredio
as well) I noticed that in UDL fb_deferred_io_init() is called from udl_fb_open(),
i.e. .fb_open call-back (in other words every time user-space app opens /dev/fb0)
while in QXC this is done only once and much earlier in qxlfb_create(), which is
called with .fb_probe call-back. So moved fb_deferred_io_init() in UDL driver from
udl_fb_open() to udlfb_create() which is also called from .fb_probe.

With that change I finally got video output via fbdev from user-space app,
but only on the first run. The next attempt to run inevitably ends with
kernel crash showing the following stack-trace (having half of the new screen
rendered on display):
------------------------>8-------------------------
Stack Trace:
  udl_handle_damage+0x48/0x210
  udl_crtc_mode_set+0x6ee/0x754
  drm_crtc_helper_set_mode+0x25e/0x438
  drm_crtc_helper_set_config+0x6d6/0x814
  __drm_mode_set_config_internal+0x48/0xc8
  drm_mode_setcrtc+0x320/0x478
  drm_ioctl+0x22c/0x3e4
  SyS_ioctl+0xa4/0x8cc
  EV_Trap+0x108/0x10c
random: crng init done
------------------------>8-------------------------

I'm wondering if UDL driver (its DRM flavor) was ever tested for
fbdev in user-space? If so and it really works for somebody
maybe I'm doing something terribly wrong - in that case any comments
are very welcome.

One interesting note: it seems like pure DRI stuff works like a charm.
I may run kmscube perfectly fine without any changes whatsoever and do it
many times in a row.

Thanks in advance,
Alexey