I'm another one of those poor souls who is getting no signal on mDP -> DP with the P2715Q.
Usually I am able to get it working again through some combination of unplugging the DP cable / rebooting / power cycling / setting to MST secondary, but now I just can't get it to work again (cold boot). I'm pretty much at the end of my tether, I have spent hours debugging this and trying to get it to work. Note that I've had this problem on and off since it first arrived.
Again, this is not a driver or OS issue, as I can't even get into the BIOS. I'm using the supplied mDP -> DP cable with an NVIDIA GTX 780 Ti. The DP port on the graphics card works with other monitors (same cable).
The monitor revision printed on the back of the unit is A01. I have scoured the internet for any kind of firmware update available, but I assume it's something that is only done in-house at Dell.
I have run the monitor diagnostics but they don't seem to have any function other than identify dead pixels and/or test that the panel works.
I purchased the monitor through Amazon (in the future I think I will always purchase through Dell), and have filled in my details to associate the service tag with my name as outlined here: en.community.dell.com/.../11682.monitor-warranties-and-replacement-instructions;cid=266889&lid=5129275
I'm not sure what my next step is here - contact Dell support?