How to force X to use a specific GPU / output

Polytropon freebsd at
Sat Nov 30 20:02:45 UTC 2019

With a HP Pavilion dm-1 "netbook" laptop I got, I have a problem
since I connected a HDMI display: There are now several green
pixels on the screen, in areas where the color is nearly black.
They also appear in videos and make the "compression squares"

Note: Those are _not_ defective pixels on the display.

When the system boots up, everything is fine. As soon as the
system enters X, color isn't displayed properly as before.
The video output on both HDMI and VGA, even at the same time,
shows the colors correctly.

During searches related to that problem, I once found an image
that looked like what I'm seeing, and I read an article, even
though regarding a Macbook, that this problem occurs due to
internal switching between graphics hardware depending on if
the internal screen should be driven or an external one. Sadly,
I cannot find any of those again...

However, I found this:

You can see "green clouds" around the curves, and the colors
look slightly distorted. Everything that is "nearly black" now
looks green - the _same_ green. The image above shows that
problem on something that looks like "Windows 7".

In the laptop here, there is an AMD CPU with graphics APU, if
my information regarding the device is correct. From "pciconf -lv":

vgapci0 at pci0:0:1:0:     class=0x030000
                        card=0x1611103c chip=0x98021002 rev=0x00 hdr=0x00
    vendor     = 'Advanced Micro Devices, Inc. [AMD/ATI]'
    device     = 'Wrestler [Radeon HD 6310]'
    class      = display
    subclass   = VGA

For X, xf86-video-ati and xf86-video-radeon packages are installed.
X automatically loads a working driver; from /var/log/Xorg.0.log:

[    25.256] (II) LoadModule: "ati"
[    25.256] (II) Loading /usr/local/lib/xorg/modules/drivers/
[    25.257] (II) Module ati: vendor="X.Org Foundation"
[    25.257]    compiled for 1.18.4, module version = 19.0.1
[    25.257]    Module class: X.Org Video Driver
[    25.257]    ABI class: X.Org Video Driver, version 20.0
[    25.259] (II) LoadModule: "radeon"
[    25.259] (II) Loading /usr/local/lib/xorg/modules/drivers/
[    25.276] (II) Module radeon: vendor="X.Org Foundation"
[    25.276]    compiled for 1.18.4, module version = 19.0.1
[    25.276]    Module class: X.Org Video Driver
[    25.276]    ABI class: X.Org Video Driver, version 20.0
[    25.282] (II) RADEON: Driver for ATI/AMD Radeon chipsets:
        AMD Radeon HD 6310 Graphics, AMD Radeon HD 6250 Graphics,
[    25.293] (==) RADEON(0): Depth 24, (--) framebuffer bpp 32
[    25.293] (II) RADEON(0): Pixel depth = 24 bits stored in 4 bytes (32 bpp pixmaps)
[    25.293] (==) RADEON(0): Default visual is TrueColor
[    25.293] (==) RADEON(0): RGB weight 888
[    25.293] (II) RADEON(0): Using 8 bits per RGB (8 bit DAC)
[    25.293] (--) RADEON(0): Chipset: "AMD Radeon HD 6310 Graphics" (ChipID = 0x9802)
[    26.364] (II) RADEON(0): Output LVDS has no monitor section
[    26.364] (II) RADEON(0): Output HDMI-0 has no monitor section
[    26.370] (II) RADEON(0): Output VGA-0 has no monitor section
[    26.370] (WW) RADEON(0): 3 ZaphodHeads crtcs unavailable. Some outputs will stay off.
[    26.381] (II) RADEON(0): Output LVDS connected
[    26.381] (II) RADEON(0): Output HDMI-0 disconnected
[    26.381] (II) RADEON(0): Output VGA-0 disconnected

When I copied those messages, only the laptop's display was active with
a size of 1366x768, which is its exact physical size. When connecting
a HDMI or VGA display (I'm using a TV which offers all those inputs),
the output is colorful and exact, as it should be. I assume LVDS refers
to the laptop's own screen.

I also had X create a configuration file. The relevant section
is this one:

Section "Device"
        Identifier  "Card0"
        Driver      "radeon"
        BusID       "PCI:0:1:0"

So there seems to be only _one_ "graphics card", even though there
are three outputs (internal, VGA analog, HDMI).

What worries me most:

The display was _fine_ before connecting a HDMI TV as external monitor!

Combing back to what I once read (sorry, no source available, I cannot
find it anymore): A suggestion was to use a specific tool (on the Mac)
to force one of the two graphic "cards" to make the problem go away.

Is this possible with X? If yes, what specific settings need to be
applied? Any suggestions what to try? What to search for - maybe this
kind of problem has a specific name?

I have no idea what I should do, other than abusing this laptop as a
poor desktop (external display, keyboard, mouse)...

Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...

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