[coreboot] VBT versions compatibility with different FSP/GOP versions

[Paul Menzel]

Dear coreboot folks,

I noticed commit 5d1494adda44 (mb/system76/tgl: Update VBTs to version 
250) [1]:
> mb/system76/tgl: Update VBTs to version 250
>
> Commit 4c7e97b26a34 ("Update fsp submodule to upstream master branch")
> included an update to the VBT from 240 to 250, breaking parsing of
> existing VBTs.
>
> After that commit, the VBT was parsed as (from gaze16-3060-b):
> [DEBUG]  PCI: 00:02.0 init
> [INFO ]  GMA: Found VBT in CBFS
> [INFO ]  GMA: Found valid VBT in CBFS
> [INFO ]  framebuffer_info: bytes_per_line: 4096, bits_per_pixel: 32
> [INFO ]                     x_res x y_res: 1024 x 768, size: 3145728 at 0xd0000000
> [DEBUG]  PCI: 00:02.0 init finished in 6 msecs
>
> When the expected output is:
> [DEBUG]  PCI: 00:00:02.0 init
> [INFO ]  GMA: Found VBT in CBFS
> [INFO ]  GMA: Found valid VBT in CBFS
> [INFO ]  framebuffer_info: bytes_per_line: 7680, bits_per_pixel: 32
> [INFO ]                     x_res x y_res: 1920 x 1080, size: 8294400 at 0xd0000000
> [DEBUG]  PCI: 00:00:02.0 init finished in 6 msecs
>
> Generate blobs for the new version using Intel Display Configuration
> Tool (DisCon) v3.3, based on the existing 237 and 240 VBTs.
>
> (For our edk2 payload, the UEFI GOP driver was updated to 17.0.1077.)

Updating the submodule pointer of 3rdparty/fsp brings in commit 
849ce8261bb0 (Tiger Lake FSP A.0.7E.70) with no commit message body. The 
diff contains:

TigerLakeFspBinPkg/Client/SampleCode/Vbt/Vbt.json

@@ -23891,7 +24584,7 @@
                     },
                     {
                       "WidgetType": "Label",
      -                "WidgetName": "VBT Version: 240",
      +                "WidgetName": "VBT Version: 250",^M
                       "Visibility": "",
                       "Data": [],
                       "HelpText": "",

Vladimir (φ-coder) also published the tool intelvbtupgrader [2] to 
address this problem.

I would have assumed that FSP versions would be backwards compatible. I 
attache the output of `intel_vbt_decode` of both VBT files.

Does somebody have more insight?

Kind regards,

Paul

[1]: https://review.coreboot.org/c/coreboot/+/82246
[2]: https://review.coreboot.org/c/coreboot/+/82722