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Mac failed Nvidia fix

So first some background.  Do_you_have_a_mirror gave me a mid 2010 Macbook Pro of unknown condition.  Installed an ssd with Ubuntu 18.04 and 16.04 dual boot.  I found that Ubuntu 18.04 would boot to a black screen while 16.04 would boot fine.  Now I can't remember if 18.04 could boot in single user or if I had to chroot from 16.04.  

Now this machine has Nvidia discrete graphics chip plus the Intel graphics built into the cpu.  I tried installing the proprietary drivers and it didn't work.  So it seems the Nvidia chip is faulty.  

Well, after searching the web, I found there is a utility for Mac OS.   And a little more digging found instructions for Linux.  

The fix is to turn off power to the Nvidia chip in the EFI variables in the Nvram.  In Linux the nvram is mounted at /sys/firmware/efi/efivars  

In Linux (If you can boot -- maybe from an Arch iso):  

umount /sys/firmware/efi/efivars/

mount -t efivarfs rw /sys/firmware/efi/efivars

printf "\x07\x00\x00\x00\x01\x00\x00\x00" > /sys/firmware/efi/efivars/gpu-power-prefs-fa4ce28d-b62f-4c99-9cc3-6815686e30f9

# In my case the last command shows an error but is actually successful.  

Evidently it is even easier from Mac OS:  

Boot to single user by holding Command+s

sudo nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00

Hope this helps, Thomas

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