Broken display; snow stripes ghosts using DisplayLink & MacBook Pro i5 i7 15/17 inch

[Bevan]

Many people are having trouble with 15 and 17 inch MacBook Pros (MBP) with i5 and i7 processors when there is no external monitor plugged in — not by the USB DisplayLink adapter nor the Mini DisplayPort (MDP) (i.e. with a DVI or VGA adaptor).

I've spent several hours testing and debugging this as well as reading through bug reports from other people on this forum and thought that this summary would be helpful for others.

This bug causes the MBP's display to have snow, stripes, random patterns or other graphics that render the display unusable when the following criteria are all TRUE;

1. 15 and 17 inch 2010 model MBPs — Intel i5 or i7 processors
2. Mac OS X 10.6 — Snow Leopard
3. DisplayLink driver is installed — any 1.6 beta version (probably 1.5 too)
4. Nothing is plugged into the MBP's Mini DisplayPort (MDP) — even an adapter with no monitor attached will stop the bug from occurring
5. System Preferences > Energy Saver > Automatic graphics switching is on — this is turned on by default in OS X
6. OS X booted in normal mode — not safe mode (shift) or single-user mode (command+s)
7. No applications are using advanced graphics rendering — iMovie, iPhoto, Adobe Flash player, many third party apps
8. (Possibly also; When using external monitors the MacBook Pro's screen is not the primary monitor — the monitor with the menu bar)

This can occur when booting (a moment after the login window appears) or when the computer is already on and any of the above change such that they all become TRUE. E.g. Something is unplugged from the MDP, or "Automatic graphics switching" is turned on.

I have simply explain the technical nature of the bug below, but if you just want a quick fix, any one of the following should fix or workaround the issue;

• Turn off System Preferences > Energy Saver > Automatic graphics switching
• Plug something (anything) into the Mini DisplayPort (MDP) — just an adapter will fix it, it doesn't need to be attached to a monitor
• Install gfxCardStatus and configure it to only use NVIDIA — this may not fix the issue when booting
• Uninstall the DisplayLink driver

The issue is related to features in OS X 10.6 and Intel's i3, i5 and i7 processors (CPUs). These CPUs have a "lightweight" (low performance, low power usage) graphics processor unit (GPU) built into them. This lightweight GPU consumes a lot less energy than the seperate "heavyweight" (high performance, power hungry) GPUs which were traditionally the only GPU in high-end MBPs.

The i5 and i7 MBPs ship with and use both heavyweight and lightweight GPUs (in i5 and i7 MBPs, these are "NVIDIA GeForce GT 330M" and "Intel HD Graphics" respectively), and OS X 10.6 is able to seamlessly switch between them to save energy and extend battery life when processing power is not needed.

The MBP will always use the lightweight GPU unless any of the following are true;

• The MDP is in use — more processing power is needed to power more pixels
• An application that needs heavyweight graphics processing is in use — iMovie, iPhoto and many others
• "Automatic graphics switching" is off — this disables the GPU-switching feature

However, the GPU switching in itself however is not the issue. The USB DisplayLink device uses a "software-screen" — that means a screen which does not have hardware which the OS can interact with about directly. OS X 10.6 has a bug which prevents software-screens and the lightweight GPU from working together, even when the software-screen's hardware component is physically disconnected. (Only the DisplayLink software-screen has a hardware component. Most software-screens have no hardware component.)

Most of the suggested workarounds above only prevent the MBP from switching to the lightweight GPU. This means that if you use one of these workarounds, the MBP will consume about 11% more power than it would if you uninstalled the DisplayLink driver, unplugged anything from the MBP's MDP and turn "Automatic graphics switching" on.

So turning "Automatic graphics switching" off is a good workaround for almost all use-cases. However you may wish to uninstall the DisplayLink driver and turn "Automatic graphics switching" on if you are expecting to need longer battery life (such as on a long flight) and are not using graphics-heavy applications.

The real fix for this bug is up to Apple. August 17's Snow Leopard Graphics update changed some of the symptoms of this bug, but did not fix it or reduce it's severity. The bug is reproducible with or without the update.