rMBP13 Late 2013 with Dell P2415Q, only 40 Hz on OS X 10.9.5

[Amethyst1]

Hello,

this is my first post here. I'm using a 13-inch Retina MacBook Pro (Late 2013) with Intel Iris 5100 graphics running OS X Mavericks (10.9.5) with all available updates and have connected a Dell P2415Q monitor, configured to SST mode, using the supplied mini-DP-to-DP cable. As expected, the system defaults to 3840×2160 at 30 Hz.

I have attempted to increase the refresh rate by using SwitchResX 4.5.2. However, I can only get up to 40 Hz using CVT-RB timings; aiming for a higher refresh rate causes all screens to go black and I have to hold the power button to turn off the MacBook. 40 Hz is marginally better than 30, e.g. I can see the mouse cursor is a little less laggy/jumpy than before.

When using Linux, I can get 3840×2160 at 52 Hz or 3520×1980 at 60 Hz using the exact same system, cable and monitor without any problems. These two modes take full advantage of the pixel clock limit which is 450 MHz for Haswell-ULT according to Intel. The monitor's OSD confirms it is operating at these modes when I set them and the mouse cursor movement etc. is much smoother than at 30 Hz (in fact, I see little difference between 52 Hz on the P2415Q and 60 Hz on my other monitors).

These tests have been performed with the MacBook's internal display being enabled and the P2415Q connected to the Thunderbolt port closest to the MagSafe port, nothing was plugged into the second TB port or HDMI (so two displays in total).

As I've not tried Yosemite or El Capitan yet, this problem may be due to Mavericks (my experiments have proven it's not a hardware problem) but has someone else come across this oddity?

 

[Amethyst1]

I apologise for replying to my own thread, but I think I've gained an important insight.

On Linux, the following modes work fine (pixel clock reported by the utility I used to generate the modes):

Resolution Refresh Pixel Clock ------------------------------ 3840×2160..52 Hz...460.25 MHz (?) 3520×1980..60 Hz...449.75 MHz

Those are the expected results given the pixel clock limit imposed by the hardware's DisplayPort implementation.

Whereas on OS X 10.9.5, the limit I can reach is substantially lower, as indicated by the following modes I am able to get to work (pixel clock reported by SwitchResX):

Resolution Refresh Pixel Clock ------------------------------ 3840×2160..40 Hz...352.06 MHz 3408×1917..50 Hz...349.81 MHz 3104×1746..60 Hz...351.56 MHz

3840×2160 at 41 Hz (resulting in 361 MHz) doesn't work. And while the increased refresh rates are seriously nice, reducing the resolution (but having the screen upscale it) causes text to look blurry. The positive thing is that I can get by with 3104×1746 on 23.8" without turning on HiDPI

So, the question can be rephrased as: why is the pixel clock limit on OS X lower than the hardware's constraints?

 

[RoboWarriorSr]

If I remember correctly, OS X has an artificial limit, I believe windows should be able to do at least 52 hertz. Since you are on Mavericks, there was a tweak here that increased the clock speed on display port allowing 52 hertz. As for why, it's probably another Apple thing they may take out later, I mean this is not the first time they have done this before.

 

[Amethyst1]

Thanks for your reply, I'm aware of the pixel clock patch but thought it only removes the limitation concerning HDMI. However, it seems it may also allow higher clocks via DisplayPort. I'm going to give this a try