Arm backend: implement EthosUBackend::is_available() hardware check

backends/arm/runtime/EthosUBackend.cpp

@@ -76,8 +76,7 @@ class EthosUBackend final : public ::executorch::runtime::BackendInterface {
   ~EthosUBackend() = default;
 
   virtual bool is_available() const override {
-    // TODO: revise to use a register check/init function
-    return 1;
+    return platform_is_available();
   }
 
   Result<DelegateHandle*> init(

backends/arm/runtime/EthosUBackend_Cortex_A.cpp

@@ -323,6 +323,23 @@ Error invoke_linux_driver(
 }
 } // namespace
 
+bool platform_is_available() {
+  // Open the NPU device driver; thrown if it is absent or unopenable.
+  const LinuxDriverOptions defaults;
+  try {
+    EthosU::Device device(defaults.device_path.c_str());
+    (void)device;
+    return true;
+  } catch (const std::exception& e) {
+    ET_LOG(
+        Info,
+        "Ethos-U device %s not available: %s",
+        defaults.device_path.c_str(),
+        e.what());
+    return false;
+  }
+}
+
 PlatformState* platform_init(
     ArrayRef<CompileSpec> specs,
     MemoryAllocator* allocator) {

backends/arm/runtime/EthosUBackend_Cortex_M.cpp

@@ -54,6 +54,21 @@ namespace arm {
 
 struct PlatformState {};
 
+// TODO: Upstream ethosu_driver_is_registered() into the core driver.
+// Keep the weak fallback until all builds are confirmed to be using a driver
+// with ethosu_driver_is_registered() implemented, at which point the weak
+// fallback can be removed.
+extern "C" __attribute__((weak)) bool ethosu_driver_is_registered(void) {
+  return true;
+}
+
+bool platform_is_available() {
+  // Check whether ethosu_init() was called before ExecuTorch runs.
+  // If ethosu_init() was not called and this unconditionally returned true,
+  // then the firmware freezes without any error.
+  return ethosu_driver_is_registered();
+}
+
 PlatformState* platform_init(
     executorch::runtime::ArrayRef<executorch::runtime::CompileSpec> /*specs*/,
     executorch::runtime::MemoryAllocator* /*allocator*/) {

backends/arm/runtime/EthosUBackend_Internal.h

@@ -77,6 +77,9 @@ extern unsigned char* ethosu_fast_scratch;
 extern size_t ethosu_fast_scratch_size;
 }
 
+// Returns whether the Ethos-U device driver is available
+bool platform_is_available();
+
 PlatformState* platform_init(
     executorch::runtime::ArrayRef<executorch::runtime::CompileSpec> specs,
     executorch::runtime::MemoryAllocator* allocator);