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Old 11-21-2021, 08:53 PM   #1
Registered: Oct 2006
Location: Leiden, Netherlands
Distribution: LFS, Ubuntu Hardy
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How to replace VBT for i915?

Apparently a couple years ago a patch was committed to change the VBT ( ). But I cannot find any information on how to actually use this. Specifically, what needs to be enabled in the kernel build? And how exactly does one replace the VBT, and do I presume it needs done in the initrd?

For some background, computers running Libreboot do not have VBT implemented. Although Coreboot seems more receptive to VBT ( ) it is still binary data that, though documented, cannot be easily created from scratch. Certain laptops running Libreboot have low contrast on the LCD, though not on external monitors. With a colorimiter I have measured the contrast to be 50:1, and on the stock bios it is somewhere around 250:1. I'd like to grab the VBT from a laptop with stock bios, load it temporarily with libreboot, and see if this has any effect.
Old 11-22-2021, 09:07 AM   #2
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If you found it as a patch, just update the software. Any patch should be in the next release. Speaking as a HD4000 owner, any card using the i915 will still be good for little or nothing with or without any VBT patch.
Old 11-25-2021, 07:04 PM   #3
Registered: Oct 2006
Location: Leiden, Netherlands
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Well this doesn't look very promising. Apparently the VBT is supposed to be exposed at /sys/kernel/debug/dri/0/i915_opregion/i915_vbt. For my kernel this 'debug' directory is empty. Apparently what is supposed to happen is you somehow copy this (how, I'm not sure) to an actual file that you put in /lib/firmware in your initrd. Then, apparently, you add i915.vbt_firmware=i915_vbt (or whatever file name you gave it in the initrd) to the kernel boot parameters. (I got that last part off of a bug report and not any official documentation).

The problem is, I cannot get the VBT firmware without running the stock bios. I have a spare defective motherboard that I flashed back to stock using an external programmer. But I cannot even get through booting up a live Slackware DVD (which runs a lot newer of a kernel then I use personally), before that damned management engine shuts me down. So either I buy another motherboard or I'm SOL, I guess. Unless there is some way to unpack the stock bios *.ROM file (which I got by copying the contents of the chip using an external programmer), but I doubt that's even possible.

Having better LCD contrast for me is in the "it would be nice" pile. These laptops in fact do run otherwise fine without any VBT at all, though that is only by chance. Typically for photo editing I use an external display, but it doesn't get dark enough. So I have to shine lights toward the monitor from behind where I sit, and hope that I don't cast a shadow.


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