Add hexagon dsp faq

docs/faq.md

@@ -50,3 +50,14 @@ Why is the UI getting poor responsiveness when running model with GPU runtime?
 Try to set `limit_opencl_kernel_time` to `1`. If still not resolved, try to
 modify the source code to use even smaller time intervals or changed to CPU
 or DSP runtime.
+
+Why is MACE not working on DSP?
+------------------------------------------------------------------------------
+Running models on Hexagon DSP need a few prerequisites for DSP developers:
+
+* You need make sure SOCs of your phone is manufactured by Qualcomm and has HVX supported.
+* You need a phone that disables secure boot (once enabled, cannot be reversed, so you probably can only get that type phones from manufacturers)
+* You need sign your phone by using testsig provided by Qualcomm. (Download Qualcomm Hexagon SDK first, plugin your phone to PC, run scripts/testsig.py)
+* You need install Hexagon nnlib backend by following nnlib README (https://github.com/XiaoMi/nnlib).
+
+Then, there you go. You can run Mace on Hexagon DSP.
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