Analyzing the iMac 5K Retina display: How do you get 5K @ 60Hz from a last-gen GPU? (updated)

Last week, Apple announced a new 27-inch iMac that packs an impressive 5K Retina display. As we’ve already detailed, these new 5K displays are thinner, cheaper, draw less power thanks to a more-efficient LED backlight, and, perhaps most importantly, Apple is selling the whole 27-inch iMac system at a mind-blowing price of $2500. That’s the same price tag on Dell’s 5K monitor.
Updated November 4: Sadly, it’s time for a mea culpa. On October 24 we mistakenly identified the iMac with 5K Retina display as having a 30Hz panel. The 27-inch iMac does indeed output 5K @ 60Hz — and to hit those kinds of frequencies, Apple used its own customized timing controller, rather than DisplayPort 1.2 (which can’t drive 5K @ 60Hz). More on that can be found further down the story.

Original story: A late-2014 display hooked to an early 2012 GPU

The GPU powering the 27-inch iMac with Retina 5K display is the R9 M290X, with the R9 M295X offered as an optional upgrade. The R9 M295X hasn’t technically been announced, but rumors from months back suggested this would be a Tonga-class GPU. Regardless, the R9 M290X is the minimum spec — and that chip is a rebranded HD 8970M, which was a rebranded HD 7970M, which is functionally equivalent to a desktop Radeon HD 7870.
In other words, the GPUs inside the new iMac are going to be limited to DisplayPort 1.2. That matters, because it takes roughly 17.2Gbps of bandwidth to drive a 4K @ 60 fps signal in a single stream (Single Stream Transport). To summarize the difference between SST and MST, an MST display creates two half-width tiles on the monitor and interleaves two different DisplayPort streams together to create a contiguous image, while an SST display functions like a standard monitor. MST and SST displays typically look identical in common applications, but some games support MST poorly, resulting in menus or functions crammed into half the monitor, or movies playing back in a squashed, half-width format.

Critically, however, MST is the only way to drive a larger-than-4K panel. DisplayPort 1.2 has just enough bandwidth to support a single 4K @ 60 fps SST stream, but 5K is far too large for the standard. When Apple talks about a 40-gigabit TCON, it may have designed a single TCON to output to two DP 1.2 streams — that’s not technically impossible — but it’s not being done with a single stream within the DP 1.2 spec.
Read: No, TV makers, 4K and UHD are not the same thing
Since Tonga doesn’t support HDMI 2.0 or DP 1.3 (which does support 5K SST), DP 1.2 is the only available standard to piggy-back. If Apple had somehow redesigned the TCON to compress a 5K stream into existing DisplayPort 1.2 bandwidth, it wouldn’t need a 40Gbps TCON in the first place. Anandtech notes that there’s another possibility — Apple may indeed have designed its own TCON, overclocked it, customized it for low overhead timing, and be pulling just enough bandwidth out of DP 1.2 to get it done.
Updated November 4: An iFixit teardown shows that Apple did indeed use a customized TCON — a modified Parade Technologies part using a non-standard 60-pin Embedded DisplayPort (eDP) connector (it’s usually just 40 pins). We’re not sure if Apple is using a variant of eDP 1.3, or some own Frankensteinian solution that combines two eDP 1.2 feeds — but in either case, there’s clearly enough bandwidth to drive a 5K display at 60Hz.

This kind of issue is common with MST — but did Apple overclock DP to hit higher bandwidths and avoid MST?

Either way, Apple would have to overclock the DisplayPort signal by 50-100% to hit the bandwidth it needs for 5K on single stream transport.

Refresh rates, gaming, and scaling

The many clever layers of the iMac with Retina display

Scaling is another potential issue — but Apple has always done this one better than Microsoft and 5120×2880 has exactly four times the pixels of the old 2560×1440 monitors, which should make scaling up relatively simple.
Finally, gaming — and here’s where the reality is going to bite. You aren’t going to be doing any gaming on a 5K display at anything like high detail levels. You may not even pull it off at low detail levels, and for a very simple reason: The R9 M290 is a midrange GPU from 2012 boxing way, way out of its weight class on this one. Despite the term, 5K is not 25% more pixels than 4K — it’s almost two times as many pixels.
Not even dual GTX 980s in SLI could drive 60-fps high-detail gaming on that kind of rig. And that means there’s no chance any AMD mobile GPU — even if the R9 M295 is a Tonga implementation — is going to be able to do it either.

http://www.extremetech.com/gaming/192305-analyzing-the-imac-5k-retina-display-how-do-you-squeeze-5k-out-of-a-last-gen-gpu