External DP Not Detected on MacBook Pro mid-2015 w/Jessie

[c64whiz]

I bought an Apple mini-DP to VGA adapter. In MacOS (Sierra) it works fine with the VGA adapter provided by the classroom. However, in Linux, no such luck. (It seems the ports are actually "Thunderbolt" but mini-DP compatible.)

Details: the classroom has a Extron device connected to the PC video card which splits the signal to an Extron switchbox and to the PC's monitor (basically a 'Y'). Then, from the switchbox, the cables go across the classroom to a projection system.

When I plug the VGA cable from the PC (cable to the Extron 'Y') Linux detects DP1 just fine, meaning 'xrandr' shows me modes for DP1. However, if I use the VGA cable from the switchbox (which is what IT intends for us to use)...nothing. 'xrandr' thinks DP1 is 'disconnected'. FWIW, Connecting the same mini-DP-VGA adapter directly to a VGA monitor at home, works fine in both Linux and Mac.

As for MacOS...the VGA cable from the switchbox (which IT wants us to use) also works just fine. The problem is only in Linux.

It's a mid-2015 MacBook Pro Retina w/onboard video. 'lspci' says its an "VGA compatible controller: Intel Corporation Crystal Well Integrated Graphics Controller". The onboad 'eDP1' device (Retina display) is fine. I have Debian Jessie installed with a backported kernel (Linux myhost 4.8.0-0.bpo.2-amd64 #1 SMP Debian 4.8.11-1-bpo8+1 (2016-12-14) x86_64 GNU/Linux) to support the Wifi drivers (also giving me a bit 'o' trouble, but that's for another post...)

Any ideas?

TIA.

--C64

[Head_on_a_Stick]

'lspci' says its an "VGA compatible controller: Intel Corporation Crystal Well Integrated Graphics Controller".

I think we need more information:

Code: Select all

# update-pciids
lspci -nn | grep VGA

If that is a Skylake device (or similar) then you may need the non-free firmware:

https://packages.debian.org/jessie-back ... sc-nonfree

Or perhaps a newer graphics stack will help:

https://forums.bunsenlabs.org/viewtopic.php?id=1185

[c64whiz]

This is definitely a path I've not tried and am anxious to (especially the xorg-intel package from Backports!). I won't be able to play with the kernel until this weekend (I want to image the drive in case something goes wrong).

On the Skylake matter, is there way to tell if it is a Skylake device (or not)?

Thank again!!

--C64

[Head_on_a_Stick]

is there way to tell if it is a Skylake device (or not)?

Code: Select all

# update-pciids
lspci -nn | grep VGA

[c64whiz]

Duh, sorry. Although I still don't know if I have a Skylake processor. The 'lspci -nn' returns:

Code: Select all

00:02.0 VGA compatible controller [0300]: Intel Corporation Crystal Well Integrated Graphics Controller [8086:0d26] (rev 08)

Also, I added the additional/new Graphic stack. I'll have to test later this week.

Will let you know, and thanks for the help.

--C64

[c64whiz]

Apparently I left this unanswered and I don't like that. So my apologies for the really late response, but the solution to the VGA adapter not working was power. Yes, as in electricity. For some reason, the MBP did not put out enough juice through the VGA for it to show up properly. Once I bought an HDMI to VGA adapter which uses it's own power source via micro-usb, VGA display worked fine.