Sources close to the hip-hop producer tell TMZ ... his support for DMX is unwavering and has been there long before the tax evasion case. We're told Swizz plans to visit DMX in prison and send him all the positive energy he can muster.

We're also told the 2 were working on a music project together that's obviously hit some snags, but when X is released after his 1-year sentence ... it's go-time, and they plan to finish the album.

Our sources say Swizz also went out of his way to speak on DMX's behalf to officials involved in his federal tax case, trying to convince them he's a good guy with human problems. S.B. is following in the footsteps of X's manager and Jayson Williams, who have also gone to bat for the rapper.