[yocto] [meta-raspberrypi] Screen(s) remain black on Raspberry Pi 3 B+

[Bernhard Mayritsch]

Hi!

I'm facing some trouble with the X-Server on the Raspberry Pi 3 B+. No
matter if I use a screen on the HDMI port or running the 7" touch screen,
they remain black. After testing with sumo - where at least a splash screen
was shown - I moved on to thud, as recommended by Andrei Gherzan to sort
out the error "No screen found." The error is gone, but nothing is shown on
the screen, not even a splash screen. Switching the branches of poky,
meta-openembedded and meta-raspberrypi to latest master still shows the
same issue.

In order to make sure not to waste time on some hardware issue, I gave
Raspbian a try; both screens (Raspberry Pi Touchscreen & HDMI) work fine
there.

The entire build is pretty much default. The MACHINE is set to
"raspberrypi3-64" and I added:
ENABLE_UART = "1"
RPI_USE_U_BOOT = "1"
... to get the debug interface working and to use U-Boot which at least
shows some lines on the screen during the early boot steps. There are no
custom recipes or meta layers yet.
I have built core-image-x11 and core-image-sato, which both show the same
behaviour.

Xorg.0.log shows the following:

[    14.767] (II) LoadModule: "fbdev"
[    14.777] (WW) Warning, couldn't open module fbdev
[    14.777] (EE) Failed to load module "fbdev" (module does not exist, 0)

[    15.393] (II) modeset(0): EDID for output HDMI-1
[    15.393] (II) modeset(0): EDID for output Composite-1
[    15.393] (II) modeset(0): Printing probed modes for output Composite-1
[    15.393] (II) modeset(0): Modeline "720x480"x31.3   13.50  720 734 798
858  480 483 486 502 interlace (15.7 kHz e)
[    15.393] (II) modeset(0): Output HDMI-1 disconnected
[    15.393] (II) modeset(0): Output Composite-1 disconnected
[    15.393] (WW) modeset(0): No outputs definitely connected, trying
again...
[    15.393] (II) modeset(0): Output HDMI-1 disconnected
[    15.393] (II) modeset(0): Output Composite-1 connected
[    15.393] (II) modeset(0): Using sloppy heuristic for initial modes
[    15.393] (II) modeset(0): Output Composite-1 using initial mode 720x480
+0+0
[    15.393] (==) modeset(0): Using gamma correction (1.0, 1.0, 1.0)
[    15.393] (==) modeset(0): DPI set to (96, 96)

It complains to be unable to load the frame buffer driver (is it used on
VC4?) and correctly detects the 7" screen with its 720x480.

The X-Server seems to be up anyways:
root at raspberrypi3-64:/var/log# ps|grep -i xorg
  318 root      3700 S    xinit /etc/X11/Xsession -- /usr/bin/Xorg :0 -br
-pn
  324 root      113m S<   /usr/bin/Xorg :0 -br -pn

As it's the first time I use Yocto to build a X11 image (only worked on
headless systems until now) I think to miss something here.

Does anyone have a hint  on that?
Any help is greatly appreciated!

Thanks in advance,
Mac
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