2012-09-10: Linux on a MacBook Air and Nvidia Projector Woes

When I was quite young, in middle school, I switched from Windows to Linux. I initially used Red Hat, but fairly quickly converted to Debian. Eventually, when OS X first came out I was very excited and switched to the Mac for my personal machines.

However, more recently, OS X has been becoming less and less Unix-like and my tastes for Apple-ism having been reduce, so in December (after classes were over), I switched back to Linux. First, I used Ubuntu and then I switched to Arch.

However, running Linux on a laptop can be exciting, and a MacBook Air with an Nvidia graphics card is particularly exciting. I had a brutal problem with my projector setup, but I found the solution. In this post, I lay out my tale of woe.

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Since it’s September again, it is time to teach class and I need to project from my laptop. I’d been using multiple displays for a long time with my Mac (I have an external screen in my office), so I presumed that I had everything working correctly.

I use disper to manage my display configurations. From Linux’s perspective, my Air has three displays: DFP-0 (the external display when it is connected with DVI), DFP-1 (the external display when it is connected with VGA), and DFP-2 (the built-in screen.) So I assumed it would be as simple as typing disper -c after connecting to clone my desktop across the two displays.

Unfortunately, I got the error message: "Displays do not share a common resolution". That’s strange, I think, I distinctly remember using these projectors last year, on OS X and getting a decently sized screen (1024x768, I believe.) A quick disper -l reveals that my DFP-1 only has one resolution: "640x480".

This was right before class, so I just used that resolution only on the projector and looked up at the screen when I needed to. Ugly, but workable.

The next day I read all about XRandr and how you can add video modes using gtf and xrandr and thought I had everything figured out. When I went to connect again during a day I didn’t teach, I had an unbearable time of getting tons and tons of error messages and no success.

I dealt with the small screen in class for a second day.

After that, I decided that I would connect with OS X on my laptop and write down which video modes, resolutions, refresh rates, etc it could handle so I could pass the correct arguments to gtf. I was delighted when I realized I’d be able to run the projector at 1400x1050 and my laptop screen at 1440x900 and have them share a 1400x900 desktop. This wasn’t the default in OS X, but it was an option.

Booting back to Linux, I had a harrowing experience trying to override the X server settings to get this mode enabled. Nothing.

At this point, I realized that I originally chose disper rather than xrandr because Nvidia cards haven’t always fully supported xrandr, but used another "meta mode" system instead. I decided to use a meta mode:

  nvidia-settings --assign CurrentMetaMode="DFP-2: 1440x900 { ViewPortIn=1400x900, ViewPortOut=1400x900+20+0 }, DFP-1: 1400x1050 { ViewPortIn=1400x900, ViewPortOut=1400x900+0+75 }"

Unfortunately, this failed too, and would sometimes crash my X server. But I was able to at least mirror the screen on the projector on my laptop, but it was a small resolution (640x480) and it was unscaled on my computer, so it was a tiny little box:

  nvidia-settings --assign CurrentMetaMode="DFP-2: 1440x900 { ViewPortIn=640x480, ViewPortOut=640x480+400+210 }, DFP-1: 640x480 { ViewPortIn=640x480, ViewPortOut=640x480+0+0 }"

But I continued to try...

I inspected the logs and found that I got the error message "Unable to read EDID for display device DFP-1". A quick run to Wikipedia told me that EDID is the format for giving the valid display frequencies for devices... so maybe OS X was reading it correctly but Linux wasn’t?

I found an OS X app that would save EDID data and saved mine for the projector (after booting into OS X.) Unfortunately after parsing it, it said that it was fake data and that actually the EDID couldn’t be read. Defeated again.

This was a fruitful path though, because it made me discover the ModeValidation option in my X config where I could add AllowNonEdidModes so that the X server wouldn’t insist on only using modes that were given by the EDID block.

Failure.

Next, I tried to turn off other checks on valid modes:

NoHorizSyncCheck

Failure.

NoVertRefreshCheck

Failure.

NoDFPNativeResolutionCheck

Failure.

I had almost given up. I decided to read through the entire Nvidia X configuration manual and I discovered the ModeDebug option that would give detailed reasons for why certain modes were not allowed. I turned that on, restarted X, and tried to use the big resolution...

The error message was "pixel clock exceeds maximum EDID pixel clock". I don’t know what a pixel clock is, but I looked in the manual and found another mode validation override:

NoEdidMaxPClkCheck

Success.

Now, I can use my projector at the full resolution. And it’s beautiful.

It’s days like this that make you proud to be a Linux user. I’ve subdued the computer and made it do exactly what I want. It feels glorious.

The only remaining problems I have with the laptop are... the microphone doesn’t work and sometimes (maybe once every three weeks) the X server will segfault, normally right after coming back from sleep. This last thing is the worst because there’s no way to reset the video settings, so I have to reboot. The saga will continue...