embedded3 R6-1: the device geometry default ON (RTX_E3_GEOM_DEVICE=0 restores the host build); a body without a device form of φ falls back to the host, logged once, nothing allocated

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/body.rs

@@ -153,9 +153,10 @@ impl Body {
         self
     }
 
-    /// R6-1: attach the device form of φ (`RTX_E3_GEOM_DEVICE=1` evaluates
-    /// it on the band instead of the host closure; the caller guarantees
-    /// the two are the same arithmetic).
+    /// R6-1: attach the device form of φ (the device geometry, default ON,
+    /// evaluates it on the band instead of the host closure; the caller
+    /// guarantees the two are the same arithmetic). A body without one
+    /// falls back to the host build.
     #[must_use]
     pub fn with_device_sdf<F>(mut self, f: F) -> Self
     where
@@ -165,6 +166,12 @@ impl Body {
         self
     }
 
+    /// Whether the body carries a device form of φ.
+    #[must_use]
+    pub fn has_device_sdf(&self) -> bool {
+        self.device_sdf.is_some()
+    }
+
     /// The device form of φ at `t`, when the body has one.
     #[must_use]
     pub fn device_sdf(&self, t: f64) -> Option<DeviceSdf> {

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/device.rs

@@ -163,7 +163,7 @@ pub struct DeviceStep {
     cut: Option<cut::DeviceCut>,
     /// Steps since the multigrid hierarchy was last rebuilt (moving bodies).
     steps_since_hierarchy: usize,
-    /// R6-1: the persistent device cut geometry (`RTX_E3_GEOM_DEVICE=1`).
+    /// R6-1: the persistent device cut geometry (default ON; `RTX_E3_GEOM_DEVICE=0` off).
     geom: Option<geom::DeviceGeom>,
 }
 

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/device/cut.rs

@@ -746,10 +746,20 @@ impl DeviceStep {
             let mut field = self.host_field.take().unwrap_or_else(|| Field::new(g));
             self.download_for_rebuild(&mut field);
             let l_down = lap.elapsed();
-            // R6-1 (`RTX_E3_GEOM_DEVICE=1`): the end-of-step cut geometry on
-            // the device, its face tables written into the persistent
-            // predictor set; the host classifies the mirror.
-            if super::geom::enabled() {
+            // R6-1 (default ON, `RTX_E3_GEOM_DEVICE=0` off): the end-of-step
+            // cut geometry on the device, its face tables written into the
+            // persistent predictor set; the host classifies the mirror. A
+            // body the device cannot build falls back to the host (logged
+            // once, nothing allocated).
+            let geom_on = super::geom::enabled()
+                && match super::geom::unsupported(&self.solver) {
+                    Some(reason) => {
+                        super::geom::log_fallback(reason);
+                        false
+                    }
+                    None => true,
+                };
+            if geom_on {
                 if let Some(dc) = self.cut.as_mut() {
                     let geom = self
                         .geom

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/device/geom.rs

@@ -1,5 +1,7 @@
 //! R6-1: the body's φ and the cut geometry on the device (`e3_geom.cu`),
-//! behind `RTX_E3_GEOM_DEVICE=1`. The corner φ (the narrow band's
+//! default ON since 2026-09-24 (`RTX_E3_GEOM_DEVICE=0` restores the host
+//! build; a body without a device form of φ falls back to the host, logged
+//! once). The corner φ (the narrow band's
 //! keep-or-evaluate, φ from the body's `DeviceSdf` polyline), the face
 //! apertures and face-centre φ (written straight into the persistent
 //! `DeviceCut` predictor apertures and distances), the cell volumes and
@@ -82,9 +84,36 @@ struct GeomGrid {
 unsafe impl DeviceRepr for GeomGrid {}
 unsafe impl ValidAsZeroBits for GeomGrid {}
 
-/// `RTX_E3_GEOM_DEVICE=1`.
+/// The device geometry: default ON (R6-1's two-period 3D gate held
+/// 2026-09-24); `RTX_E3_GEOM_DEVICE=0` restores the host build.
 pub(super) fn enabled() -> bool {
-    std::env::var("RTX_E3_GEOM_DEVICE").is_ok_and(|v| v == "1")
+    !std::env::var("RTX_E3_GEOM_DEVICE").is_ok_and(|v| v == "0")
+}
+
+/// Why the device geometry cannot build this solver's body (the host
+/// builds it instead), or `None` when it can.
+pub(super) fn unsupported(solver: &Solver) -> Option<&'static str> {
+    if solver.params.wall_scheme != WallScheme::CutCell {
+        return Some("the wall scheme is not the cut cell");
+    }
+    if solver.params.aperture_substeps != 0 {
+        return Some("aperture substeps are on");
+    }
+    match solver.body() {
+        None => Some("no body"),
+        Some(b) if !b.has_device_sdf() => Some("the body has no device form of φ"),
+        Some(_) => None,
+    }
+}
+
+/// The host fallback, logged once per process.
+pub(super) fn log_fallback(reason: &str) {
+    static LOGGED: std::sync::Once = std::sync::Once::new();
+    LOGGED.call_once(|| {
+        eprintln!(
+            "    R6-1 device geometry: host fallback ({reason}); the cut geometry is built on the host"
+        );
+    });
 }
 
 /// The persistent device geometry: corner φ and bound, the evaluated

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/mod.rs

@@ -297,7 +297,7 @@ pub struct Solver {
     /// `(setup ns, iterate ns, solves, CG iterations)` summed.
     poisson_profile: (u64, u64, u64, u64),
     /// R6-1: the next moving rebuild's cut geometry, built on the device
-    /// (`RTX_E3_GEOM_DEVICE=1`); `build_mask` classifies it instead of
+    /// (default ON, `RTX_E3_GEOM_DEVICE=0` off); `build_mask` classifies it instead of
     /// evaluating φ on the host.
     pub(super) pending_cut: Option<CutGeometry>,
     /// R6-1: retired device-built geometry arrays (empty on the host path)