NVIDIA broken AGAIN

I love NVIDIA's proprietary binary driver. It gives me so many hours of troubleshooting fun. Here's the latest installment:

Ubuntu 8.10 Kernel upgrade 2.6.27-9 = No more EDID from my monitor

Without the EDID information from the monitor, NVIDIA has a meltdown and only lets me have 640x480... sometimes... Other times it just blows up.

After several hours of hacking with the xorg.conf in an attempt to replicate the modelines properly, I have an epiphany.. Why not FAKE the EDID!

Option "CustomEDID" "DFP-0:/etc/X11/edid.bin"

This allows me to give NVIDIA a bin file and tell it to use this instead of asking my monitor for the EDID... So now I just pull the EDID from the monitor and we're done, right?

Download read-edid 2.0

> sudo get-edid > edid.bin

VBE/DDC service about to be called
Read EDID

Performing real mode VBE call
Interrupt 0x10 ax=0x4f15 bx=0x1 cx=0x0
Function supported
Call failed

The EDID data should not be trusted as the VBE call failed

The EDID from get-edid on amd64 systems comes out with a bad checksum! NVIDIA won't load an EDID with a bad checksum! AGGHHH!!

But wait... read-edid provides us with another tool... parse-edid... Normally this will spit out xorg.conf like data regarding your monitor specifics.. but it also has a bit of code that reads the 8 bit checksum.... but it doesn't tell us what the checksum IS... YET.

open up parse-edid.c and navigate to line 249.

this bit:
MSG( "EDID checksum failed - data is corrupt. Continuing anyway." );
Needs to look like this:
printf( "EDID checksum failed - data is corrupt. Continuing anyway. %d", checksum );

And recompile.

Then, when we run parse-edid against the bin we got out of get-edid.. we see this:

EDID checksum failed - data is corrupt. Continuing anyway. 19

That 19 is important. The last hex value in the .bin file needs to NEGATE this number.. It needs to equal -19 as a signed 8 bit integer...

Grab your favorite hex editor and make that very last value in the file equal to the opposite of your checksum value!

wait, what's -19 in hex?

Well, it's an 8 bit checksum.. so.. add -19 to the max positive signed 8 bit value of 256 and we get 237... now convert 237 into hexadecimal..

The Lazy way

So now I stick ED into the last bit of the file, stick in in the xorg.conf, and viola. No more EDID problems. Works just like before.

Piece of cake, right?