docs: NVIDIA-INSTRUCTIONS.md: how to test the NVIDIA fix on the real PC, read the results and undo it

.no-release-yet

@@ -1,4 +1,4 @@
 This branch is not ready to be released: do not merge it into the release branch.
 When the feature is complete and tested, ask the user whether it may be released;
 on a yes, delete this file, then open the pull request into the release branch.
-Open work and how to pick it up: TODO-nvidia.md. Research notes: NVIDIA-RESEARCH.md.
+Open work and how to pick it up: TODO-nvidia.md. Research notes: NVIDIA-RESEARCH.md. How to test on the NVIDIA PC: NVIDIA-INSTRUCTIONS.md.

NVIDIA-INSTRUCTIONS.md

@@ -0,0 +1,86 @@
+# Testing the NVIDIA fix on a real PC
+
+For the PC with an RTX 5080 (or any RTX 20 series or newer card) that shows a corrupted picture when
+gaming mode (gamescope) starts. Branch `feature/nvidia-gaming-fix`, not released. Background and open
+work: `TODO-nvidia.md`; research with sources: `NVIDIA-RESEARCH.md`.
+
+## What the fix does
+With a supported NVIDIA GPU (RTX 20 series or newer, same check as the VRAM booster) and its driver installed it
+- adds `nvidia-drm.modeset=1 nvidia-drm.fbdev=1` to the kernel command line (Limine, systemd-boot or GRUB;
+  the file is backed up as `<file>.bak-gamescope-wizard`),
+- loads the NVIDIA modules in the initramfs, but only while every installed kernel has them (a pacman hook,
+  `/etc/pacman.d/hooks/85-steamify-nvidia-initramfs.hook`, decides again at every kernel change),
+- rebuilds the initramfs and boot entries and checks that the parameters really are in them.
+It takes effect after a reboot. Older cards (GTX 10 series and older) are left alone.
+
+## Before you start
+- CachyOS with the NVIDIA driver installed (`nvidia-open`), a user with sudo, `git`.
+- Be at the PC and able to see the screen: the last step needs your eyes.
+- Know the way back (below) in case the PC doesn't boot to a picture: a TTY (Ctrl+Alt+F3) or the boot menu's
+  other kernel entry.
+
+## Steps
+1. Get the code:
+   ```
+   git clone -b feature/nvidia-gaming-fix https://github.com/theupriser/steamify-cachyos
+   cd steamify-cachyos
+   ```
+   (An existing checkout: `git fetch && git checkout feature/nvidia-gaming-fix && git pull`.)
+2. Look first, change nothing:
+   ```
+   bash tests/nvidia-hardware-test.sh check
+   ```
+3. Apply the fix (asks for your sudo password), then **reboot**:
+   ```
+   bash tests/nvidia-hardware-test.sh apply
+   ```
+4. After the reboot, check again:
+   ```
+   bash tests/nvidia-hardware-test.sh check
+   ```
+5. Start gaming mode, look at the screen, then (from the desktop or a TTY):
+   ```
+   bash tests/nvidia-hardware-test.sh visual
+   ```
+   It asks whether the picture was clean and for one line on what you saw.
+6. Send back `~/steamify-nvidia-report.txt` (every run adds to it), or start a new session in the checkout and
+   say "read TODO-nvidia.md and the report".
+
+(`./steamify.sh` from the same checkout also applies the fix, automatically, as part of the SteamOS conversion:
+there is no menu option for it yet. The script above shows more.)
+
+## Reading `check`
+| Line | Good result | If not |
+|---|---|---|
+| `PASS a supported NVIDIA GPU ... is detected` | your 5080 | `FAIL ... older than RTX 20`: the card is skipped on purpose. `FAIL no NVIDIA GPU with its driver`: driver not installed (nouveau?) |
+| `NVIDIA PCI device id 0x... (not on a legacy list ...)` | RTX 20 or newer | on a legacy list: older card |
+| `no chwd legacy lists found` | (note only) | the check cannot tell old from new cards; every card counts as supported. Tell me, it's open item 4 in `TODO-nvidia.md` |
+| `PASS nvidia-drm.modeset=1 is on the running kernel's command line` (and `fbdev=1`) | after step 3 and the reboot | "in the boot loader's file but not booted yet": reboot, or the boot entry used is another one |
+| `PASS nvidia_drm modeset = Y` (and `fbdev = Y`) | after the reboot | `unreadable`: it needs sudo, enter the password |
+| `... early-load drop-in is used / skipped` | note only: skipped means a kernel without the NVIDIA modules is installed | nothing to do |
+
+## Reading the result
+- Parameters on, `modeset = Y`, `fbdev = Y`, picture clean: the fix works.
+- Parameters on and `Y`, picture still corrupted: not this fix. The report has the gamescope and NVIDIA log lines and
+  the versions of `nvidia-open`, `nvidia-utils`, `gamescope` and `gamescope-session-cachyos`: the cause is probably the
+  gamescope/driver combination for Blackwell (see `NVIDIA-RESEARCH.md`).
+- No picture at all after the reboot: undo it (next section) from a TTY or the other kernel entry, and send the report.
+
+## Undo
+Turning the SteamOS conversion off in the wizard removes the fix too. On its own, from the checkout:
+```
+SCRIPT_DIR=$PWD bash -c 'source lib/common.sh; source lib/hdmi-refresh.sh; source lib/vram-booster.sh; source lib/nvidia.sh; nvidia_disable'
+```
+It removes the parameters, the early-load file, the pacman hook and its script, and rebuilds the boot entries (reboot after).
+By hand: delete ` nvidia-drm.modeset=1 nvidia-drm.fbdev=1` from `/etc/default/limine` (`KERNEL_CMDLINE[default]`),
+`/etc/sdboot-manage.conf` (`LINUX_OPTIONS`) or `/etc/default/grub` (`GRUB_CMDLINE_LINUX_DEFAULT`), or restore the
+`.bak-gamescope-wizard` copy next to it; remove `/etc/mkinitcpio.conf.d/90-steamify-nvidia.conf`,
+`/etc/pacman.d/hooks/85-steamify-nvidia-initramfs.hook` and `/usr/local/libexec/steamify-nvidia-initramfs`; then
+`sudo limine-mkinitcpio` (Limine) or `sudo mkinitcpio -P` and `sudo sdboot-manage gen` / `sudo grub-mkconfig -o /boot/grub/grub.cfg`.
+
+## What is and isn't proven
+Proven in VMs (QEMU/KVM, CachyOS ISO 260809): the parameters land and survive a reboot on Limine, systemd-boot and GRUB;
+disabling restores the original; on Limine the pacman hook keeps the early-load file right across kernel reinstalls
+(with fake NVIDIA modules). Not proven: that the fix cures the 5080's picture, the real NVIDIA modules in the
+initramfs, DKMS hook order, systemd-boot/GRUB with a pacman transaction, and the RTX 20+ check against a real chwd list.
+Unit test (no hardware): `bash tests/nvidia-test.sh`.

TODO-nvidia.md

@@ -1,6 +1,6 @@
 # TODO: NVIDIA fix for gaming mode (branch `feature/nvidia-gaming-fix`)
 
-Delete this file, `NVIDIA-RESEARCH.md` and `.no-release-yet` when the feature is released.
+Delete this file, `NVIDIA-INSTRUCTIONS.md`, `NVIDIA-RESEARCH.md` and `.no-release-yet` when the feature is released.
 Written 2026-10-01 so a new session can pick this up.
 
 ## Problem
@@ -70,7 +70,8 @@ needs a test on a real older card. Design for the toggle: on by default for any
 what it does, `tests/nvidia-hardware-test.sh` reports GPU and driver so results can be compared.
 
 ## Still to do
-1. [ ] **On the NVIDIA PC** (only place the real fix can be judged), from this checkout:
+1. [ ] **On the NVIDIA PC** (only place the real fix can be judged; the full steps, how to read the results and the undo
+   are in `NVIDIA-INSTRUCTIONS.md`), from this checkout:
    `bash tests/nvidia-hardware-test.sh check` (before), `... apply` (asks sudo), reboot,
    `... check` (after: `nvidia_drm modeset/fbdev = Y`), boot gaming mode, then
    `... visual` (was the picture clean?). Everything goes to `~/steamify-nvidia-report.txt`: