The Orin camera works: it was a missing overlay, not dead hardware

An IMX219 is live on the Orin's CAM0 at /dev/video0, verified by capture rather
than by a bound driver: a 16,163,840-byte frame, exactly 3280x2464x2 for
10-bit Bayer, 99.8% non-zero across 40 distinct values. Real sensor data, not
an allocated buffer.

⚠ The fault was configuration, and it is indistinguishable from dead hardware.
The Pi auto-detects cameras; the Jetson does not. Until the sensor's device-tree
overlay is selected there is no /dev/video0, no sensor line in dmesg and nothing
on i2c — the same evidence a broken cable produces, which is how it got blamed
on a cable here. Fixed with config-by-hardware.py, and the README now says to
check the OVERLAYS line BEFORE suspecting hardware.

Two traps recorded with it: the -n flag needs the header number (2= for the CSI
connector) or it defaults to the 40-pin header and reports the module as
unsupported, and v4l-utils is not installed by default so v4l2-ctl reports
"command not found", which reads as "no camera".

The Pi 5 fault was genuinely a cable and is kept for contrast, along with why
its lit IR LEDs proved nothing: the illuminator sits on the 3.3V rail
independently of the i2c and CSI lanes, so it lights whenever the ribbon carries
power, even when the traces detection needs are broken.

AGENTS.md also gains the rules that came out of a 27-attempt loop in this repo:
write files with apply_patch and never write_stdin, never invent or increment a
session id, stop after two identical failures instead of retrying, and emit tool
calls as JSON arguments rather than leaking <parameter=...> markup into strings.

README.md

@@ -39,28 +39,74 @@ The same applies to inference. On the Orin, face recognition should go through T
 the GPU or DLA. On the Pi 5 it is CPU-only and will not keep up with a live stream at full
 resolution. Treat the Pi as proof the *pipeline* works, never as evidence the *performance* works.
 
-## ⚠ Current state: no camera is connected anywhere
+## Current state: working camera on the Orin
 
-Nothing is built yet, and no working camera has been attached to either board.
+**An IMX219 is live on the Orin Nano's CAM0**, confirmed by capture, not just by a bound driver:
 
-The first module, on the Pi 5, was **not detected at all**:
+```console
+$ v4l2-ctl --list-devices
+vi-output, imx219 9-0010 (platform:tegra-capture-vi:1):
+	/dev/video0
+
+$ v4l2-ctl -d /dev/video0 --stream-mmap --stream-count=1 --stream-to=/tmp/frame.raw
+$ ls -l /tmp/frame.raw
+16163840        # 3280 x 2464 x 2 bytes — full sensor, 10-bit Bayer
+```
+
+The frame was 99.8% non-zero across 40 distinct values, so it is real sensor data rather than an
+allocated buffer. Formats: `RG10` (10-bit Bayer) at 3280x2464/21fps, 1920x1080/30fps, 1640x1232/30fps.
+
+### ⚠ The Orin needed a device-tree overlay — it does NOT auto-detect
+
+This cost a whole debugging session, because the symptom is identical to a dead camera: no
+`/dev/video0`, no sensor lines in `dmesg`, nothing on i2c.
+
+**The Pi auto-detects cameras. The Jetson does not.** `camera_auto_detect=1` has no equivalent — the
+sensor's overlay must be selected explicitly, and until it is, a perfectly good camera is invisible:
+
+```bash
+sudo /opt/nvidia/jetson-io/config-by-hardware.py -l                     # list modules per header
+sudo /opt/nvidia/jetson-io/config-by-hardware.py -n 2="Camera IMX219-A" # header 2 = 24-pin CSI
+sudo reboot                                                            # required
+```
+
+⚠ **`-n` needs the header number** (`2=` for the CSI connector). Without it the tool defaults to the
+40-pin header and fails with `No configuration found for Camera IMX219-A on Jetson 40pin Header!`,
+which reads like the module is unsupported.
+
+Naming: **`-A` is CAM0, `-C` is CAM1.** `extlinux.conf` is backed up before the change, and the
+result is one `OVERLAYS` line — remove it and reboot to undo.
+
+⚠ **`v4l2-ctl` is not installed by default** (`sudo apt install v4l-utils`), and its absence reports
+as `command not found`, which is easy to misread as "no camera".
+
+### The Pi 5 camera is still dead
+
+Separate fault, genuinely hardware: not detected, traced to a bad cable with the module possibly
+damaged too. **Do not generalise the Orin's fix to it** — the Pi's auto-detect had nothing to
+configure, so an overlay is not the answer there.
+
+Evidence, for contrast with the Orin's *configuration* fault above:
 
 ```console
 $ rpicam-hello --list-cameras
 No cameras available!
 ```
 
-Diagnosed to hardware, not software. The imaging pipeline was up (`pisp_be` loaded, `/dev/media0-2`
-present), but `/sys/bus/i2c/devices/` held only `i2c-13` and `i2c-14` — **no camera i2c bus was
-instantiated and no CFE bound**, and `dmesg` had no sensor probe lines. `camera_auto_detect=1` loads
-a sensor overlay when it finds something, so an absent bus means the firmware found nothing to probe.
-Unchanged across a reboot.
+The imaging pipeline was up (`pisp_be` loaded, `/dev/media0-2` present), but
+`/sys/bus/i2c/devices/` held only `i2c-13` and `i2c-14` — **no camera i2c bus and no CFE bound** —
+and `dmesg` had no sensor probe lines. On the Pi, `camera_auto_detect=1` loads an overlay when it
+finds something, so an absent bus means the firmware found nothing to probe. Unchanged across a
+reboot, with the module's IR LEDs lit.
 
-**Confirmed a cable fault; the module itself may also be damaged.** A second module is being tried on
-an Orin Nano.
+⚠ **Lit IR LEDs prove power, not a working sensor.** The illuminator is wired to the 3.3V rail
+independently of the i2c and CSI lanes, so it lights whenever the ribbon is seated well enough to
+carry power — while a creased or partly-seated cable can still have broken exactly the i2c traces
+detection depends on. Which is what happened.
 
 ⚠ **Do not treat `/dev/video*` as evidence of a camera.** Those nodes exist on both boards with
-nothing attached — on the Pi 5 they are the codec and ISP blocks.
+nothing attached — on the Pi 5 they are the codec and ISP blocks, on the Orin
+`tegra-camrtc-ca`/`/dev/media0` is the VI platform block.
 
 ### Checking a connection
 
@@ -71,10 +117,14 @@ dmesg | grep -iE 'imx|ov5647|cfe'
 ls /sys/bus/i2c/devices/          # a camera bus should appear
 
 # Orin Nano
+grep -i OVERLAYS /boot/extlinux/extlinux.conf   # ⚠ CHECK THIS FIRST — no line, no camera
 v4l2-ctl --list-devices
 dmesg | grep -iE 'imx|camera|argus|vi:'
 ```
 
+On the Orin, check the overlay **before** suspecting hardware. An unconfigured Jetson and a dead
+camera look exactly alike.
+
 Cable notes worth keeping, since they cost a module here:
 
 - The **Pi 5 uses the narrow 22-pin FPC**; Pi 4-era modules ship with a **15-pin** cable and need the
@@ -85,7 +135,8 @@ Cable notes worth keeping, since they cost a module here:
 
 ## Planned stages
 
-1. **Capture** — get a sensor detected on the target, grab a still, establish resolution and format.
+1. ~~**Capture** — get a sensor detected on the target, grab a still, establish resolution and
+   format.~~ **Done on the Orin** (IMX219, `/dev/video0`, `RG10`, full frame captured).
 2. **Capture abstraction** — one interface, a backend per platform, chosen at runtime.
 3. **Live feed** — MJPEG first, because it works in any browser with no negotiation. WebRTC later
    only if latency demands it.