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

is_available() returned 1 unconditionally.
The consequence differs by platform:
- Cortex-M: if ethosu_init() was never called, execute() blocks
  silently inside ethosu_reserve_driver() with no error or log.
- Cortex-A: if /dev/ethosu0 is absent, execute() logs
  a driver error but does not clearly indicate the device is unavailable.

Changes:
- Cortex-M: ethosu_driver_is_registered() checks registered_drivers
  and guarantees semaphore count is > 0, so ethosu_reserve_driver() will not block.
- Cortex-A: constructs EthosU::Device to probe the default device path,
  which throws if the device is absent/unopenable.
- ethosu_driver_is_registered() is added to the Corstone SDK driver via
  examples/arm/ethos-u-setup/core_driver/ patch (patch_repo mechanism)
  and to the Zephyr hal_ethos_u driver via examples/arm/ethos-u-setup/hal_ethos_u/
  patch (git apply in zephyr/CMakeLists.txt). A weak symbol fallback
  (return true) covers build paths not reached by either patch.

Signed-off-by: Youngsik Yang <vacu9708@gmail.com>

Author: vacu9708

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 +54,19 @@ namespace arm {
 
 struct PlatformState {};
 
+// Weak fallback in case the patch to ethosu_driver_is_registered() is not
+// applied
+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.
+  // TODO: Upstream ethosu_driver_is_registered() into the core driver and
+  // remove the weak fallback.
+  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);

backends/arm/scripts/corstone_utils.cmake

@@ -53,6 +53,14 @@ function(fetch_ethos_u_content ETHOS_SDK_PATH ET_DIR_PATH)
       "source backends/arm/scripts/utils.sh && patch_repo ${ETHOS_SDK_PATH}/core_software ${core_software_base_rev} ${patch_dir}"
     WORKING_DIRECTORY ${ET_DIR_PATH}
   )
+  # Patch the Ethos-U core driver.
+  set(core_driver_base_rev "26.02")
+  execute_process(
+    COMMAND
+      bash -c
+      "source backends/arm/scripts/utils.sh && patch_repo ${ETHOS_SDK_PATH}/core_software/core_driver ${core_driver_base_rev} ${patch_dir}"
+    WORKING_DIRECTORY ${ET_DIR_PATH}
+  )
   # Always patch the core_platform repo since this is fast enough. TODO:
   # examples/arm/ethos-u-setup/core_platform/0002-*.patch and 0003-*.patch are
   # transient bridges that guard Armv8-M-only MPU init and the Armv7-M-and-newer

examples/arm/ethos-u-setup/core_driver/0001-Add-ethosu_driver_is_registered.patch

@@ -0,0 +1,54 @@
+From 08111f0f4f5034237181b8ac4b50ccdc84575fb9 Mon Sep 17 00:00:00 2001
+From: Youngsik Yang <vacu9708@gmail.com>
+Date: Thu, 4 Jun 2026 18:09:17 +0900
+Subject: [PATCH] Add ethosu_driver_is_registered() non-blocking availability
+ check
+
+Expose whether ethosu_init() has been called by reading registered_drivers
+under the mutex. This allows callers to check driver availability without
+blocking, unlike ethosu_reserve_driver() which waits indefinitely.
+
+Signed-off-by: Youngsik Yang <vacu9708@gmail.com>
+---
+ include/ethosu_driver.h | 8 ++++++++
+ src/ethosu_driver.c     | 8 ++++++++
+ 2 files changed, 16 insertions(+)
+
+diff --git a/include/ethosu_driver.h b/include/ethosu_driver.h
+index 1d7a573..9f609c3 100644
+--- a/include/ethosu_driver.h
++++ b/include/ethosu_driver.h
+@@ -370,6 +370,14 @@ struct ethosu_driver *ethosu_reserve_driver(void);
+  */
+ void ethosu_release_driver(struct ethosu_driver *drv);
+ 
++/**
++ * Returns true if at least one driver has been registered via ethosu_init().
++ * Non-blocking; safe to call before reserving a driver.
++ *
++ * @return true if a driver is registered, false otherwise.
++ */
++bool ethosu_driver_is_registered(void);
++
+ /**
+  * Static inline for backwards-compatibility.
+  *
+diff --git a/src/ethosu_driver.c b/src/ethosu_driver.c
+index 8f39a4b..1e1caaf 100644
+--- a/src/ethosu_driver.c
++++ b/src/ethosu_driver.c
+@@ -797,3 +797,11 @@ void ethosu_release_driver(struct ethosu_driver *drv)
+     }
+     ethosu_mutex_unlock(ethosu_mutex);
+ }
++
++bool ethosu_driver_is_registered(void)
++{
++    ethosu_mutex_lock(ethosu_mutex);
++    bool result = registered_drivers != NULL;
++    ethosu_mutex_unlock(ethosu_mutex);
++    return result;
++}
+-- 
+2.34.1
+

examples/arm/ethos-u-setup/hal_ethos_u/0001-Add-ethosu_driver_is_registered.patch

@@ -0,0 +1,47 @@
+From a4b521ee5841374db88af0ca6337b7741e72bf50 Mon Sep 17 00:00:00 2001
+From: Youngsik Yang <vacu9708@gmail.com>
+Date: Fri, 5 Jun 2026 02:38:21 +0900
+Subject: [PATCH] Add ethosu_driver_is_registered() non-blocking availability
+ check
+
+Signed-off-by: Youngsik Yang <vacu9708@gmail.com>
+---
+ include/ethosu_driver.h | 8 ++++++++
+ src/ethosu_driver.c     | 8 ++++++++
+ 2 files changed, 16 insertions(+)
+
+diff --git a/include/ethosu_driver.h b/include/ethosu_driver.h
+index 1d7a573..5b2c70d 100644
+--- a/include/ethosu_driver.h
++++ b/include/ethosu_driver.h
+@@ -396,3 +396,11 @@ static inline int ethosu_invoke_v2(const void *custom_data_ptr,
+ #endif
+ 
+ #endif // ETHOSU_DRIVER_H
++
++/**
++ * Returns true if at least one driver has been registered via ethosu_init().
++ * Non-blocking; safe to call before reserving a driver.
++ *
++ * @return true if a driver is registered, false otherwise.
++ */
++bool ethosu_driver_is_registered(void);
+diff --git a/src/ethosu_driver.c b/src/ethosu_driver.c
+index bd68c9e..43f8e86 100644
+--- a/src/ethosu_driver.c
++++ b/src/ethosu_driver.c
+@@ -796,3 +796,11 @@ void ethosu_release_driver(struct ethosu_driver *drv)
+     }
+     ethosu_mutex_unlock(ethosu_mutex);
+ }
++
++bool ethosu_driver_is_registered(void)
++{
++    ethosu_mutex_lock(ethosu_mutex);
++    bool result = registered_drivers != NULL;
++    ethosu_mutex_unlock(ethosu_mutex);
++    return result;
++}
+-- 
+2.34.1
+

zephyr/CMakeLists.txt

@@ -59,6 +59,35 @@ if(CONFIG_EXECUTORCH)
   endif()
   if(CONFIG_ETHOS_U)
     set(EXECUTORCH_BUILD_ARM_BAREMETAL ON)
+    # Patch hal_ethos_u to add ethosu_driver_is_registered() if not already
+    # present. Uses git apply (not patch_repo) because hal_ethos_u has no stable
+    # release tag to reset to.
+    set(_hal_ethos_u_dir "${ZEPHYR_BASE}/../modules/hal/ethos_u")
+    set(_hal_ethos_u_patch
+        "${EXECUTORCH_DIR}/examples/arm/ethos-u-setup/hal_ethos_u/0001-Add-ethosu_driver_is_registered.patch"
+    )
+    if(EXISTS "${_hal_ethos_u_dir}" AND EXISTS "${_hal_ethos_u_patch}")
+      execute_process(
+        COMMAND git apply --check "${_hal_ethos_u_patch}"
+        WORKING_DIRECTORY "${_hal_ethos_u_dir}"
+        RESULT_VARIABLE _patch_check
+        OUTPUT_QUIET ERROR_QUIET
+      )
+      if(_patch_check EQUAL 0)
+        execute_process(
+          COMMAND git apply "${_hal_ethos_u_patch}"
+          WORKING_DIRECTORY "${_hal_ethos_u_dir}"
+        )
+        message(
+          STATUS "Applied ethosu_driver_is_registered patch to hal_ethos_u"
+        )
+      else()
+        message(
+          STATUS
+            "Skipping hal_ethos_u patch - already applied or hal_ethos_u not found"
+        )
+      endif()
+    endif()
   endif()
 
   set(EXECUTORCH_BUILD_EXTENSION_RUNNER_UTIL ON)