How to resolve CGDirectDisplayID changing issues on newer multi-GPU Apple laptops in Core Foundation
- by Dave Gallagher
In Mac OS X, every display gets a unique CGDirectDisplayID number assigned to it. You can use CGGetActiveDisplayList() or [NSScreen screens] to access them, among others. Per Apple's docs:
A display ID can persist across
processes and system reboot, and
typically remains constant as long as
certain display parameters do not
change.
On newer mid-2010 MacBook Pro's, Apple started using auto-switching Intel/nVidia graphics. Laptops have two GPU's, a low-powered Intel, and a high-powered nVidia. Previous dual-GPU laptops (2009 models) didn't have auto-GPU switching, and required the user to make a settings change, logoff, and then logon again to make a GPU switch occur. Even older systems only had one GPU.
There's an issue with the mid-2010 models where CGDirectDisplayID's don't remain the same when a display switches from one GPU to the next. For example:
Laptop powers on.
Built-In LCD
Screen is driven by Intel chipset.
Display ID: 30002
External
Display is plugged in.
Built-In
LCD Screen switches to nVidia
chipset. It's display ID changes:
30004
External Display is driven
by nVidia chipset.
...at this point,
the Intel chipset is dormant...
User unplugs External Display.
Built-In LCD Screen switches back to
Intel chipset. It's display ID
changes back to original: 30002
My question is, how can I match an old display ID to a new display ID when they alter due to a GPU change?
Thought about:
I've noticed that the display ID only changes by 2, but I don't have enough test Mac's available to determine if this is common to all new MacBook Pro's, or just mine. Kind of a kludge if "just check for display ID's which are +/-2 from one another" works, anyway.
Tried:
CGDisplayRegisterReconfigurationCallback(), which notifies before-and-after when displays are going to change, has no matching logic. Putting something like this inside a method registered with it doesn't work:
// Run before display settings change:
CGDirectDisplayID directDisplayID = ...;
io_service_t servicePort = CGDisplayIOServicePort(directDisplayID);
CFDictionaryRef oldInfoDict = IODisplayCreateInfoDictionary(servicePort, kIODisplayMatchingInfo);
// ...display settings change...
// Run after display settings change:
CGDirectDisplayID directDisplayID = ...;
io_service_t servicePort = CGDisplayIOServicePort(directDisplayID);
CFDictionaryRef newInfoDict = IODisplayCreateInfoDictionary(servicePort, kIODisplayMatchingInfo);
BOOL match = IODisplayMatchDictionaries(oldInfoDict, newInfoDict, 0);
if (match)
NSLog(@"Displays are a match");
else
NSLog(@"Displays are not a match");
What's happening above is I'm caching oldInfoDict before display settings change, letting them change, and then comparing it to newInfoDict by using IODisplayMatchDictionaries(), which will say either "yes, both displays are the same!" or "no, both displays are not the same." Unfortunately, it does not return YES if GPU's have changed for a monitor. Example of the dictionary's it's comparing:
// oldInfoDict (Display ID: 30002)
oldInfoDict: {
DisplayProductID = 40144;
DisplayVendorID = 1552;
IODisplayLocation = "IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer/display0/AppleBacklightDisplay";
}
// newInfoDict (Display ID: 30004)
newInfoDict: {
DisplayProductID = 40144;
DisplayVendorID = 1552;
IODisplayLocation = "IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/P0P2@1/IOPCI2PCIBridge/GFX0@0/NVDA,Display-A@0/NVDA/display0/AppleBacklightDisplay";
}
As you can see, the IODisplayLocation key changes when GPU's are switched, hence IODisplayMatchDictionaries() doesn't work. I can, theoretically, compared just the DisplayProductID and DisplayVendorID keys, but I'm writing end-user software, and am worried of a situation where users have two or more identical monitors plugged in.
Any help is greatly appreciated! :)
