docs: nvidia/: TODO, research and test instructions for the NVIDIA fix (moved here from the steamify-cachyos branch); AGENTS.md: feature notes live in this repo

nvidia/RESEARCH.md

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+# NVIDIA and gamescope: research notes (for steamify-cachyos branch `feature/nvidia-gaming-fix`)
+
+Written 2026-10-01 for the NVIDIA fix (`lib/nvidia.sh`). Delete the `nvidia/` folder when the feature is released, or move what is still useful to steamify-cachyos' `TECHNICAL.md`.
+"Verified" means seen in this session (a VM run or a fetched page); the rest is from search results or memory.
+
+## The user's PC
+RTX 5080 (Blackwell). Blackwell only works with NVIDIA's open kernel modules (`nvidia-open`), driver 570+.
+The corrupted picture at gaming mode start is not diagnosed yet: the fix adds kernel parameters, the real cause
+may be the gamescope/driver combination. `tests/nvidia-hardware-test.sh` collects the facts.
+
+## What the fix changes and why (reasoning, not proven for gamescope)
+- gamescope runs the display itself through DRM/KMS. With NVIDIA that needs `nvidia-drm.modeset=1`; `nvidia-drm.fbdev=1`
+  gives the console a framebuffer from `nvidia-drm`, so the hand-off from the console to gamescope is clean.
+- Loading the NVIDIA modules in the initramfs (early KMS) only changes how early boot looks (native resolution,
+  splash, passphrase prompt), not speed. It is not needed for gamescope, which starts after login (believed, not proven).
+- No source found that mentions these parameters for gamescope; the fix is a standard NVIDIA/Wayland setup, applied
+  automatically. Whether it cures the 5080's picture is only known after `tests/nvidia-hardware-test.sh`.
+
+## Which cards (decision 2026-10-01: RTX 20 series or newer only, same check as the VRAM booster)
+The code skips a card whose PCI id is on chwd's legacy lists (`vram_nvidia_legacy_id`); the notes below are why older
+cards are not promised.
+- Open kernel modules: Turing (RTX 20, GTX 16) and newer only; they need the GSP processor first built into Turing.
+  Maxwell, Pascal and Volta (GTX 900/10 series, Titan V) only work with the proprietary driver, whose legacy branch
+  is 580. Source: [NVIDIA README, open kernel modules](https://download.nvidia.com/XFree86/Linux-x86_64/560.35.03/README/kernel_open.html),
+  [NVIDIA datacenter driver guide, kernel modules](https://docs.nvidia.com/datacenter/tesla/driver-installation-guide/kernel-modules.html).
+- `nvidia-drm.modeset` and `fbdev` are options of the `nvidia-drm` module in both flavours, so the fix applies to
+  any card whose driver provides `nvidia_drm` (what `nvidia_present` tests: GPU vendor 0x10de + `modinfo nvidia_drm`).
+  Whether gamescope then works on an older card is a different question, and no source says the fix helps there.
+- Not NVIDIA's proprietary driver: nouveau has no `nvidia_drm`, so the fix does nothing. The CachyOS handheld ISO boots
+  GTX 10xx and older with nouveau until the NVIDIA driver is installed
+  ([CachyOS forum](https://discuss.cachyos.org/t/information-experimental-cachyos-handheld-edition/203)).
+
+## Known gamescope problems that are not this fix
+- **GTX 1050 Ti (Pascal), driver 580.119.02, gamescope newer than 3.16.16:** the CachyOS handheld session fails to start when
+  a display is on the GPU's HDMI port (back to the TTY, loop). Workarounds: downgrade `gamescope` and `lib32-gamescope`
+  to 3.16.16, or use the motherboard's video port. Fixed upstream? unknown.
+  [CachyOS forum](https://discuss.cachyos.org/t/no-display-on-cachyos-handheld-edition-on-nvidia-with-gamescope-3-16-16/20935)
+- **VRS on PCs:** CachyOS' gamescope-session enables variable rate shading (`STEAM_USE_DYNAMIC_VRS=1`,
+  `RADV_FORCE_VRS_CONFIG_FILE`, `echo 1x1 > ...` in `/usr/lib/steamos/gamescope-session`). It broke rendering (missing
+  floors/lighting, bad post-processing) on a PC with an AMD RX 6800 XT. RADV is AMD's driver: not the NVIDIA corruption.
+  Workaround: comment those three lines, or per game `env -u RADV_FORCE_VRS_CONFIG_FILE STEAM_USE_DYNAMIC_VRS=0 %command%`.
+  [CachyOS forum](https://discuss.cachyos.org/t/cachyos-gamescope-session-asset-missing-vrs-fix/34971)
+- gamescope's DRM backend needs Vulkan DRM format modifiers; with the open Mesa driver NVK that came in Mesa 24.1
+  ([GamingOnLinux](https://www.gamingonlinux.com/2024/05/nvk-driver-gets-drm-format-modifiers-to-work-with-gamescope-in-mesa-24-1)).
+  The proprietary driver had format-modifier trouble with gamescope too (GitHub issues
+  [ValveSoftware/gamescope#1662](https://github.com/ValveSoftware/gamescope/issues/1662),
+  [#1516](https://github.com/ValveSoftware/gamescope/issues/1516): titles seen only, not read).
+
+## What was verified in VMs (QEMU/KVM, CachyOS ISO 260809)
+- Kernel parameters land and survive a reboot on Limine, systemd-boot and GRUB; disable restores the original.
+- Limine's tool is a compiled program: an appended `KERNEL_CMDLINE[default]+=" ..."` line was pasted into the kernel
+  command line as text; the fix edits the existing `KERNEL_CMDLINE[default]="..."` line.
+- `mkinitcpio` fails on a MODULES entry a kernel lacks; `limine-mkinitcpio` then skips that kernel's initramfs and boot
+  entry (parameters included); systemd-boot/GRUB image: "may not be complete" (not run with failing modules).
+- Real CachyOS/Limine pacman hooks: `10-limine-snapper-lock`, `60-limine-mkinitcpio-remove-pre`, `60-mkinitcpio-remove`,
+  `80-limine-efi-deploy`, `90-limine-mkinitcpio-remove-post`, `90-mkinitcpio-install`; Steamify's hook is `85-`
+  (DKMS's is `71-dkms-install`, not installed in the VM). A mirror older than the ISO "downgrades" kernels on reinstall.
+- The hook keeps the early-load drop-in right across kernel changes (see `nvidia/TODO.md`).
+
+## Still unknown (needs the hardware)
+- Does the fix cure the 5080's corrupted picture? Which gamescope/driver versions does the PC have?
+- Does it help, harm or do nothing on Maxwell/Pascal/Volta cards? No older card available yet.
+- Real NVIDIA modules in the initramfs (VMs only had renamed fake modules); DKMS hook order on a PC with
+  `nvidia-open-dkms`.