From a11632599145d5f18840db9f00cbec94f10cc830 Mon Sep 17 00:00:00 2001
From: Duncan Tourolle <duncan@tourolle.paris>
Date: Sun, 4 Oct 2026 20:19:04 -0400
Subject: [PATCH] Declare the DSP's RPC library, so the app can reach the
 Hexagon

QNN's Hexagon stub loads libcdsprpc.so, a vendor library, and from API 31
an app's linker namespace refuses a vendor library its manifest does not
name. QNN then fails to create its device - QNN_DEVICE_ERROR_INVALID_CONFIG,
before it reaches the DSP - and every model ran on the CPU on 0.22.0: AI
denoise took 30-131 s a photograph on the tablet.

The same engine code ran on the HTP from adb's shell, whose namespace has
no such rule, which is what hid it. With the declaration the app's probe
chose the Hexagon on the tablet and compiled every model for it. Not
required, so a device without the library still installs and runs on the
CPU. A test holds the line in the manifest.
---
 apps/darkroom-android/android/AndroidManifest.xml |  8 ++++++++
 apps/darkroom-android/src/lib.rs                  | 13 +++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/apps/darkroom-android/android/AndroidManifest.xml b/apps/darkroom-android/android/AndroidManifest.xml
index 45e42af..fd56ead 100644
--- a/apps/darkroom-android/android/AndroidManifest.xml
+++ b/apps/darkroom-android/android/AndroidManifest.xml
@@ -73,6 +73,14 @@
         android:requestLegacyExternalStorage="true"
         android:supportsRtl="true">
 
+        <!-- The DSP's RPC library, which QNN's Hexagon stub loads. From API 31
+             an app's linker namespace refuses a vendor library the manifest
+             does not name, and QNN then fails to create its device
+             (QNN_DEVICE_ERROR_INVALID_CONFIG) before it reaches the DSP:
+             every model ran on the CPU on 0.22.0. Not required, so a device
+             without one still installs and stays on the CPU. -->
+        <uses-native-library android:name="libcdsprpc.so" android:required="false" />
+
         <!-- NativeActivity rather than a Kotlin Activity: android-activity's
              glue loads libdarkroom.so and calls android_main. `android.app.lib_name`
              is how it learns which library to load, and must match [lib].name.
diff --git a/apps/darkroom-android/src/lib.rs b/apps/darkroom-android/src/lib.rs
index 6d89b05..fda94ea 100644
--- a/apps/darkroom-android/src/lib.rs
+++ b/apps/darkroom-android/src/lib.rs
@@ -538,6 +538,19 @@ mod tests {
         Some(value.to_string())
     }
 
+    /// From API 31 the linker refuses a vendor library the manifest does not
+    /// name, and QNN cannot create its Hexagon device without the DSP's RPC
+    /// library: 0.22.0 ran every model on the CPU for want of this line.
+    #[test]
+    fn the_npu_can_reach_the_dsp() {
+        assert!(
+            manifest().contains(
+                r#"<uses-native-library android:name="libcdsprpc.so" android:required="false" />"#
+            ),
+            "libcdsprpc.so must be declared, and not required"
+        );
+    }
+
     #[test]
     fn a_gallery_can_open_a_photograph_in_this_app() {
         let manifest = manifest();