How to disable a plugged-in PCI-e graphic card on OS level?

Question

I have a server running for which I want to have a possility to access it with a screen connected to VGA (very rarely, most of the time it's accessed via SSH). Therefore I have a PCI-e graphic cards plugged in and the VGA cable removed. The graphic card is passively cooled and if I open the case and touch the cool body I can feel a noticable warmth and conclue that it's consuming energy (there're no consumers close to it that could transmit the thermal energy in any way).

If I unplug the card (as suggested in Should I Disable an unused graphic card?) I have to built it in every time I want to connect a screen. I would like to avoid that as well as the energy consumption.

The de- and re-activation needs to take place on the OS level (e.g. via SSH) because otherwise I'd need a screen to configure the UEFI (or do that blindly which is no alternative) and run into a chicken-egg-problem.

I'm using Ubuntu 15.04 with Linux 4.0.2. The graphic card is labeled XFX HD 5450 850M and has a VGA, HDMI and D-SUB connector. The mainboard is an ASRock X99-Extreme without integrated graphic.

EDIT: After blacklisting used modules listed in sudo lspci -v (following @WhimsicalWombat's promising answer below) (in my case I had to use the modprobe.blacklist=module_to_blacklist kernel parameter - see https://askubuntu.com/questions/110341/how-to-blacklist-kernel-modules for more details - for radeon and snd_hda_intel) the PCIe graphic card still heats up (passive cooler has > 60 degrees and there's no heat source closeby) and no modules are listed in lspci -v for the devices. The output of lspci -v for the devices is not

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Cedar [Radeon HD 5000/6000/7350/8350 Series] (prog-if 00 [VGA controll
er])
        Subsystem: XFX Pine Group Inc. Device 303e
        Physical Slot: 4
        Flags: bus master, fast devsel, latency 0, IRQ 11
        Memory at e0000000 (64-bit, prefetchable) [size=256M]
        Memory at fbe20000 (64-bit, non-prefetchable) [size=128K]
        I/O ports at e000 [size=256]
        Expansion ROM at fbe00000 [disabled] [size=128K]
        Capabilities: [50] Power Management version 3
        Capabilities: [58] Express Legacy Endpoint, MSI 00
        Capabilities: [a0] MSI: Enable- Count=1/1 Maskable- 64bit+
        Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=010 <?>
        Capabilities: [150] Advanced Error Reporting

01:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Cedar HDMI Audio [Radeon HD 5400/6300 Series]
        Subsystem: XFX Pine Group Inc. Device aa68
        Physical Slot: 4
        Flags: bus master, fast devsel, latency 0, IRQ 10
        Memory at fbe40000 (64-bit, non-prefetchable) [size=16K]
        Capabilities: [50] Power Management version 3
        Capabilities: [58] Express Legacy Endpoint, MSI 00
        Capabilities: [a0] MSI: Enable- Count=1/1 Maskable- 64bit+
        Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=010 <?>
        Capabilities: [150] Advanced Error Reporting

EDIT 2: https://askubuntu.com/a/138953/173287 suggests to do echo 0 > /sys/bus/pci/slots/$N/power and I have a /sys/bus/pci/slots/$N where $N is number of the slot listed in lspci -v, but there's no power file.

EDIT 3: adding the modaliases of /sys/bus/pci/devices/[device]/modalias to modprobe.blacklist= kernel parameters has no effect on Ubuntu mainline kernels from http://kernel.ubuntu.com/~kernel-ppa/mainline/ and a custom 4.0.5 build with make localmodconfig and activation of all PCI options causes the "Loading initramfs [version]" screen to remain visible for ever while the graphic card is still heating up.

Answer 1

Easiest way is to blacklist and unload it's kernel module. You can see it's current module with first checking out the bus number with sudo lspci |egrep -i (vga|video) noting the first field with number like 01:00.0. Then sudo lspci -vs 01:00 |grep modules displays the module in use. For HDMI-enabled devices there's usually a subdevice like 01:00.1 which is the HDMI audio device. Blacklist that too.

Add both of those /etc/modprobe.d/blacklist or blacklist.conf preceded by blacklist command. For example "blacklist radeon" and "blacklist "snd-hda-intel" etc.

If you want to use the device you can just sudo modprobe [module name] to enable it.

If that's not enough or if you use the same module for the GPU (integrated or second adapter) you actually use, bind the one you want to disable to pci-stub driver. Best way to do this is from kernel command line at boot. Just add pcistub="pci-stub.ids=..." followed by vendorID:deviceID codes you can find with lcpci -nns (your bus number from above) pcistub="pci-stub.ids=1002:6718,1002:aa80"

If you want to get that to use after binding it to pcistub, you can unbind it via sysfs and rebind it to driver of your choice or (simpler) reboot and boot without the commandline.

I'd try first if just disabling the module autoload is enough if that's available.

EDIT:

It's possible kernel will try next available driver if you blacklist one. If so, you can blacklist that too unless you need it. It'll run out of compatible drivers soon and leave the device without driver and it should be powered down (or at least low).

If it's heating up even without a kernel driver in use, please update the post. I'd be very interested to hear that.