docs: CEC.md, which PCs have HDMI-CEC and how to add it

CEC.md

@@ -0,0 +1,48 @@
+# HDMI-CEC: does my PC have it?
+
+The **HDMI-CEC** item in Steamify lets you use Steam with your TV's remote,
+and turns the TV on and off with the PC. It only works when the PC has a
+CEC device (`/dev/cec*`). Many PCs don't: on desktop PCs CEC is rare.
+How it works: [TECHNICAL.md](TECHNICAL.md#hdmi-cec); problems:
+[TROUBLESHOOTING.md](TROUBLESHOOTING.md#the-tv-remote-does-nothing-hdmi-cec).
+
+## Graphics cards
+
+- **NVIDIA:** no. The driver doesn't expose CEC on the HDMI port, on Linux
+  or Windows.
+- **AMD and Intel:** the CEC line of the HDMI port is normally not
+  connected on desktop cards. The Linux drivers (`amdgpu`, `i915`) only
+  support CEC over a **DisplayPort** connection: some DisplayPort-to-HDMI
+  adapters and built-in converter chips pass CEC along ("DP-AUX CEC"). That
+  depends on the adapter or chip, not on the graphics card, so it's worth
+  trying a different adapter.
+
+## Devices with CEC built in
+
+- the **Valve Steam Machine** (Steamify ticks HDMI-CEC by default there);
+- some **Intel NUC** mini PCs, which have a CEC chip on the board;
+- the **Raspberry Pi**.
+
+## Any PC: a USB CEC adapter
+
+A USB CEC adapter, such as the **Pulse-Eight USB-CEC adapter** (or a
+RainShadow Tech one), sits between the PC and the TV on the HDMI cable and
+works on Linux. This is the way to get CEC on a custom build, e.g. with an
+NVIDIA card. Steamify installs what these adapters need
+(`inputattach-cec-units`), so plug it in and turn on HDMI-CEC.
+
+## Check your PC
+
+After turning on HDMI-CEC in Steamify, the menu lists the CEC devices it
+found. Or run:
+
+```bash
+ls /dev/cec*
+```
+
+No device (and no USB adapter): no CEC on this PC. Also turn on CEC on the
+TV itself: it has a brand name there (Sony: BRAVIA Sync, Samsung: Anynet+,
+LG: SimpLink, Philips: EasyLink, Panasonic: VIERA Link).
+
+This overview is based on how the Linux drivers work; hardware differs,
+so the only sure test is `ls /dev/cec*` on your own PC.

CHANGELOG.md

@@ -14,6 +14,8 @@ one per merged pull request.
     the TV turns on and off with the PC. On a Steam Machine, steamos-manager
     configures it from Steam's settings.
   - Works with any `/dev/cec*` (GPU or USB adapter); the menu lists them.
+  - `CEC.md`: which PCs have CEC (NVIDIA doesn't, AMD/Intel only over some
+    DisplayPort adapters), and USB adapters for your own build.
   - Restarts steamos-manager, which only offers Steam its HDMI-CEC settings
     when cecd was there at its start.
   - The README no longer says Steam Machine support alone gives HDMI-CEC:

README.md

@@ -29,9 +29,10 @@ Steam Machine.
    opens the newest Steamify, so you don't need the install command again.
 6. **HDMI-CEC** (experimental; off by default, on by default on a Steam
    Machine) - use Steam with your TV's
-   remote, and the TV turns on and off with the PC. Needs a GPU with CEC (like
-   the Steam Machine) or a USB CEC adapter
-   ([details](TECHNICAL.md#hdmi-cec)).
+   remote, and the TV turns on and off with the PC. Needs a PC with CEC (like
+   the Steam Machine) or a USB CEC adapter; most graphics cards, NVIDIA
+   included, don't have it. See [CEC.md](CEC.md) for which PCs do, and how to
+   add it to your own build ([details](TECHNICAL.md#hdmi-cec)).
 
 ## Steam Machine
 

TECHNICAL.md

@@ -186,7 +186,8 @@ SHA-256 in Valve's package index:
 
 It works with any `/dev/cec*`: a GPU that has CEC on its HDMI port (the
 Steam Machine; the CachyOS kernel has DisplayPort CEC built in) or a USB
-adapter. The menu lists the CEC devices found. Also turn on CEC on the TV
+adapter ([CEC.md](CEC.md) lists which PCs have one). The menu lists the CEC
+devices found. Also turn on CEC on the TV
 (Sony: BRAVIA Sync, Samsung: Anynet+, LG: SimpLink). On a Steam Machine
 `steamos-manager` writes cecd's settings from Steam's (wake the TV, put it to
 sleep), in `~/.config/cecd/config.d/`, and Steam shows its HDMI-CEC

TROUBLESHOOTING.md

@@ -45,8 +45,8 @@ journalctl --user -u cecd -b
 cec-ctl -d /dev/cec0 -S        # the devices on the TV's CEC bus (v4l-utils)
 ```
 
-No `/dev/cec*`: your GPU's HDMI port has no CEC; a USB CEC adapter
-(e.g. Pulse-Eight) works. The Steam Machine wakes up again right after going
+No `/dev/cec*`: your PC has no CEC; a USB CEC adapter (e.g. Pulse-Eight)
+works. See [CEC.md](CEC.md). The Steam Machine wakes up again right after going
 to sleep, or other devices lose CEC? That happens on SteamOS too; turn
 HDMI-CEC off in the wizard.