Successfully combined AMD APU (HD8670D) with mismatched HD7850

After lots of issues I finally got both (officially not Dual Graphics compatible) AMD GPUs working together in BOINC.

Solution :

For Windows Vista, note that AMD's latest Drivers available is the Catalyst 13.4 without Richland GPU support of the newest AMD APUs. Updated 13.4 w/Richland support, 13.8Beta1, 13.8 Beta2 and 13.10beta are Win7/8 ONLY. That means Vista users don't have a Richland display driver for now.

However, the older (Trinity) HD7660D Driver that is contained in the Cat 13.4 works just fine, it just needs to be manually installed via the Device Manager.

With Win7 or later, this is not an issue of course.

Motherboard used : AsRock FM2A75 Pro4, AMD A75 Chipset
(BIOS offers the important Options of manually Enabling OnBoard GPU and Dual Graphics, as well as assigning VRAM to it - as the APUs contained GPU needs to be forced enabled despite mismatched AMD Video Card)

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Steps required (Windows Vista)

* initially with the APU disabled, make sure Cat 13.4 is installed normally with your Video Card and that the Video Card acts as primary Display (PCI Express)

- in BIOS set Primary Display to APU (OnBoard)

- in BIOS set OnBoard GPU to Enable and manually assign VRAM (I set 2GB as I wanted to run 2 tasks in parallel)

- shutdown System and remove Video Card, connect Display to OnBoard connector

- Boot up using the APU, Vista will run with a default/safe Standard VGA Video Driver now using basic Vista Design scheme

- open Device manager and for the Display Device showing as "Standard VGA", manually install Trinity HD7660D Driver
-- the default location of the .inf File for Vista Systems :
C:\AMD\Support\13-4_vista_win7_win8_64_dd_ccc_whql\Packages\Drivers\Display\W86A_INF\CH155980.inf
(the C:\AMD Directory holds all AMD Catalyst Drivers that you installed and remains in place after installation)

- browse the device list downward to choose HD7660D ; ignore warning that the driver might be unsuitable

- reboot after being asked to do so
(at this point no exclamation point should be present on the APU Video Adapter in device manager, Catalyst Control center should show normal Hardware Info about it. If Vista Aero GUI still works in basic mode, run the Vista Performance evaluation once which should re-enable Aero Glass effects after finishing)

- shutdown System to install the HD7850 (or any other Radeon you want to use, for as long as that AMD Video Card runs off the same Catalyst Driver Package version as the APU - in this case 13.4 - it should work IMHO)

- boot up, enter BIOS and check that Dual Graphics Option is set to Enabled (that setting may not have been available with only the APU in the System)

- boot Windows and check Device Manager for both devices to show up without exclamation point

- Catalyst should display primary device operating normal and claim the 2nd Device (Video Card) Disabled. Note that you won't be able to access 2nd GPU for anything else than OpenCL applications (no crossfire, no dual display etc.)

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Technically, at this point you should have both devices fully usable to BOINC. But as the APU running off System RAM currently operates as primary display as well, we want to switch to the more potent Video Card as primary display to maximize the RAM bandwidth available to the APU (which it will have to share with CPU tasks) and not additionally burden it with the Display task.

- Restart Windows to enter BIOS again, now set Primary Display to Video Card, save settings & exit BIOS

- Plug Display chord into Video card while System is booting

- System should now run off your Video Card, with the APU available as 2nd Device for BOINC (Catalyst Control Center Hardware Info shows your Video card details as usual, lists the HD7660D as disabled device). Check the Event log after launching BOINC for presence of 2nd GPU or witness BOINC Manager showing both devices (it will visibly label them device 0 and device 1) working on a task.

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And voila, now you should have two supposedly incompatible GPUs running for ya :)

For Windows 7, manually installing the APUs driver from the device manager should not be needed (be aware that there are two Versions of the Cat 13.4 out there, one with and one without Richland support !).
After following the basic steps, it should be possible to combine an incompatible AMD Video card with your APU.

Note that I do believe the Motherboard and its BIOS plays a critical role there. It must have the option of manually forcing the APU GPU and Dual Graphics to enabled and assign amount of VRAM (if set to Auto, it's likely running off 32MB or something similar, which is typically not enough to run a GPU project, thus preventing work to be downloaded for it).
Amount of VRAM assigned depends on project(s) run, check their VRAM requirements, also remember to double the set VRAM if you plan on running 2 tasks parallel like most users do. Einstein needs 512MB VRAM available per Task if I'm not mistaken.

I've made good experiences with AsRock Motherboards not playing exactly like AMD or NVidia would want things to run (= bypass some limitations they'd like in place) and give such options to the user, don't know about other manufacturers.

The steps described are exactly in the order that got me success. You may have luck starting with Video Card already installed and set as primary device, saving you some time. However when I initially tried that, I ran into alot of severe issues including bluescreens etc.

Anyway, after weeks of experimenting, I thought this might be interesting for AMD APU owners that were frustrated like me that mismatching APU and discrete AMD GPU is not officially supported in any way.
My A10-6700 validated its first WorkUnit and it looks like it will add almost 11000 Credits/Day in performance.
I like that, that's the way it should work :)

Credits :
Thanks go to Members Crashtest and sompe of the German Planet3dNow! Forum for their help to solve the issue.
(German language Forum thread available here)