An engineer passed on an answer to me, which is as I suspect. The actual codec source code is not available.
As part of the MediaSDK 1.1 release, we have released AMF APIs, Header Files and Sample code using these APIs. The definition of the APIs are in the associated DLLs. And the source code of the DLLs is not shared.
You are right. Input is fed to the decoder using the SubmitInput API and output is read from the decoder using the QueryOutput API. For more details on the functionality please refer to the sample document simpleDecoder\docs\MediaSDK_AMF_simpleDecoder.pdf and also the AMF API reference manual AMD Media SDK\1.1\docs\amf\MediaSDK_AMF_ReferenceManual.pdf
Yes, SimpleDecoder is a sample code to show how UVD Hardware on AMD platform can be used to offload H.264 video decoding. So if you have your application with a decode call -- internally in that call, AMF APIs to offload video decoding on the UVD can be achieved.