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Scott
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dnn: silence false-positive CPU target warning for New graph engine #29457 ### Pull Request Readiness Checklist See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request * [x] I agree to contribute to the project under Apache 2 License. * [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV * [x] The PR is proposed to the proper branch (`5.x`) * [ ] There is a reference to the original bug report and related work No existing OpenCV issue or pull request directly covers this warning. This PR is the original report and fix. * [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable Not applicable. This patch changes only diagnostic behavior for an existing CPU no-op path. * [ ] The feature is well documented and sample code can be built with the project CMake Not applicable. This patch adds no feature or public API. ## Summary Suppress a misleading warning emitted when OpenCV 5 New DNN graph engine receives its default CPU target request. `FaceDetectorYN` and `FaceRecognizerSF` call: ```cpp net.setPreferableTarget(DNN_TARGET_CPU); ``` after loading their networks. When New graph engine is active, target switching is not supported. OpenCV therefore currently prints: ```text Targets are not supported by the new graph engine for now ``` However, New graph engine already executes on CPU. The CPU request does not require a target change, is ignored, and inference continues through New graph engine. The warning therefore suggests a failed configuration or fallback to Classic DNN even though neither occurs. ## Change Treat `DNN_TARGET_CPU` as a silent no-op when generic New graph engine is active. ```text New graph engine + CPU target → no target change needed → return unchanged → no warning New graph engine + non-CPU target → target remains unsupported → preserve existing warning → return unchanged ``` ## Behavior before ```text New graph engine active → caller requests CPU → request is ignored → warning emitted → inference continues on New graph engine ``` ## Behavior after ```text New graph engine active → caller requests CPU → request is ignored → no warning → inference continues on New graph engine ``` ## Unchanged behavior * New graph engine selection is unchanged. * Inference execution is unchanged. * CPU remains the effective target for generic New graph engine. * Classic DNN behavior is unchanged. * ONNX Runtime handling is unchanged. * Non-CPU targets continue to emit the existing warning. ## Motivation The current diagnostic is a false positive for the default CPU request. It reports unsupported target selection even though CPU is already the active execution target and inference succeeds through New graph engine. ## Testing * Built OpenCV locally. * Ran relevant DNN tests. * Verified New graph engine still loads and executes affected models. * Verified CPU target requests no longer emit the misleading warning. * Verified non-CPU target requests retain the existing unsupported-target warning.