On Debian, 1920x1200 support for the Talos II's AST2500 was broken between Linux 5.5.0-2 and Linux 5.6.0-1. My suspicion is that the breakage is because of Linux commit 9253f830c9166bfa6cc07d5ed59e174e9d5ec6ca. This commit added a VRAM size check to the ast driver, which considers double-buffering as mandatory. 1920x1080 resolution at 4 bytes per pixel with 2 buffers is 16.6 MB, while bumping that to 1920x1200 results in 18.4 MB. According to "lspci -v -s 0005:02:00.0", The Talos II's VRAM size is 16 MiB == 16.8 MB, which explains why 1920x1200 no longer works.
I asked on IRC, and dormito pointed out that the AST2500 supports both 16 MiB and 64 MiB VRAM size, and that in both cases, the VRAM is stolen from the BMC's RAM (512 MiB total). The default configuration is controlled by a hardware pin (no idea why Raptor picked 16 MiB), but dormito thinks it may be possible to reconfigure the AST2500's VRAM size by writing to a register on the BMC (before the host boots).
In the meantime it definitely seems like a Linux bug should be filed about this, since single-buffered 1920x1200 used to work fine but no longer does.
