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I have an ASUS Z170M PLUS motherboard and an i7 6700k CPU with integrated graphcis (Intel HD Graphics 530). I run Xubuntu 16.04.5 LTS.

I want to built a two-seat system, so I've bought a Radeon R7 240 Sapphire GPU. Both integrated and dedicated GPUs work fine when only one of them is working. However, when I try to have both active, problems begin.

I choose "Autodetect" or "PCI-E" in my BIOS settings, everything works until after the GRUB load screen. After the system starts loading, I see the message "Scanning for btrfs filesystem". The system never loads. And my root system is btrfs, I can't just delete btrfs-tools to fix that.

When I disable dual graphics support, that message is no longer there, and the system loads in moments. But I want to use both GPUs, so that is not an option.

Also, there is no scanning for btrfs with dual graphics with the iGPU set as primary, however, the dedicated graphics is not active in that instance, and I can't use it. The only thing different is when I shut the system down, both GPUs render the splash screen. So, in theory, there should be no hardware problems.

The dedicated GPU is not listed in xrandr --listproviders with the iGPU set as the primary GPU. However, it is listed in the output of lspci -v | grep -P "VGA|HDMI".

What should I do? I would prefer to use the dGPU as the primary GPU, but even using the iGPU as primary is acceptable, as long as I can get both to work at the same time.

UPD: xrandr --listproviders with two GPUs, iGPU set as primary

Providers: number : 1
Provider 0: id: 0x47 cap: 0x9, Source Output, Sink Offload crtcs: 4 
outputs: 4 associated providers: 0 name:Intel

And here's the output of lspci | grep VGA:

00:02.0 VGA compatible controller: Intel Corporation Sky Lake Integrated Graphics (rev 06)
04:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Opal XT [Radeon R7 M265]

UPD2. dGPU only

xrandr --listproviders:

Providers: number : 1
Provider 0: id: 0x54 cap: 0x9, Source Output, Sink Offload crtcs: 2 outputs: 3 associated providers: 0 name:OLAND @ pci:0000:04:00.0

lspci |grep VGA:

 04:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Opal XT [Radeon R7 M265]

UPD3. I was able to load a reserve ext4 Xubuntu 16.04.1 LTS. Both GPUs work with it, but not with my main btrfs system.

EXT4 16.04.1, dual GPUs, dGPU set as primary

xrandr --listproviders
Providers: number : 2
Provider 0: id: 0xe6 cap: 0x9, Source Output, Sink Offload crtcs: 2     outputs: 3 associated providers: 1 name:OLAND @ pci:0000:04:00.0
Provider 1: id: 0x47 cap: 0x2, Sink Output crtcs: 4 outputs: 4 associated providers: 1 name:Intel

UPD4. EXT4 16.04.1, dual GPUs, iGPU set as primary

xrandr --listproviders
Providers: number : 3
Provider 0: id: 0x84 cap: 0x9, Source Output, Sink Offload crtcs: 4 outputs: 4 associated providers: 2 name:Intel
Provider 1: id: 0x44 cap: 0x6, Sink Output, Source Offload crtcs: 2 outputs: 3 associated providers: 2 name:OLAND @ pci:0000:04:00.0
Provider 2: id: 0x44 cap: 0x6, Sink Output, Source Offload crtcs: 2 outputs: 3 associated providers: 2 name:OLAND @ pci:0000:04:00.0

Summary: my setup works as intended with an outdated-ish ext4 system. There no ATI drivers installed on it. The only difference, to the best of my knowledge, is that that system is ext4, and the one I want to work is btrfs.

UPD7. I was able to SSH into the system and reboot it without pressing reset. Turns out the system is actually fully loaded, but the message about btrfs scanning is just stuck. And I can't switch to tty either. The only thing I can do is SSH into it, basically using the computer as a headless server, or reset.

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I fixed the problem by renaming /etc/X11/xorg.conf.d/20-intel.conf to /etc/X11/xorg.conf.d/20-intel.conf~

This is the configuration it held.

Section "Device"
   Identifier  "Intel Graphics"
   Driver      "intel"
   Option      "TearFree"    "true"
EndSection

It was what interfered with dual GPUs.

Problem solved. I hope this question can help someone else.

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