available() reports whether a sensor driver was linked, instead of True

README.md

@@ -303,7 +303,11 @@ USB webcam (the board presenting as UVC) lives in
 [`usbif`](https://github.com/PyDevices/usbif)'s `examples/usbif_webcam.py`,
 which sources frames from this module when a camera is present.
 
-Known hole: [`available()` always returns True](https://github.com/PyDevices/cameraif/issues/1).
+`available()` answers whether this firmware can open a camera at all: it is
+true when at least one `esp_cam_sensor` driver was linked in, and false on a
+build with no `CONFIG_CAMERA_<chip>` enabled and on every non-P4 target. It
+does not say whether a sensor is plugged in -- that is what constructing a
+`Camera` tells you, with a message.
 
 ## Performance, measured
 

src/mod_cameraif.c

@@ -1321,8 +1321,26 @@ static MP_DEFINE_CONST_OBJ_TYPE(
     locals_dict, &cameraif_locals_dict
 );
 
+// A probe that cannot fail is not a probe. Reaching this branch already means
+// the firmware was built for a target with a MIPI-CSI controller, so what is
+// left to ask is whether any sensor driver was linked into it: esp_cam_sensor
+// drivers register themselves in the detect section walked by the open path
+// above, and with no CONFIG_CAMERA_<chip> (plus its AUTO_DETECT option) the
+// section is empty and no camera can ever be found on a perfectly good bus.
+//
+// That is not hypothetical. It is exactly the state a P4 build lands in when
+// the option is missed, and this module then reports it as "no camera sensor
+// answered on SCCB" -- a wiring-shaped error for a build-configuration cause,
+// which is the confusion micropython.cmake's note was written about and which
+// this function exists to end.
+//
+// It deliberately does NOT touch the bus. available() is asked before any pins
+// are known -- board_peripherals calls it before it passes sda/scl -- so it
+// could not probe honestly even if it wanted to, and "is a sensor plugged in"
+// is already answered, with a message, by constructing a Camera.
 static mp_obj_t cameraif_available(void) {
-    return mp_const_true;
+    return mp_obj_new_bool(&__esp_cam_sensor_detect_fn_array_start
+        < &__esp_cam_sensor_detect_fn_array_end);
 }
 static MP_DEFINE_CONST_FUN_OBJ_0(cameraif_available_obj, cameraif_available);