OK I have finally been able to configure automatic TX DMA with ping-pong buffer for both channels. Performance has improved considerably. 48 KHz sampling seems feasible with a 20% CPU load. SD card write process can have longer delays sometimes and in this cases CPU load increases over 60% or even 80% and then I get underflows in th RX DMA.
I wanted to add a data pipe but I saw on the BIOS manual that PIP module is not recommended because is going to be deprecated in future versions of the BIOS, also data pipes are not blocking, and I thought that some kind of blocking mechanism should exist in order to let other tasks take place, so I decided to use SIO module. For some reason, unknown to me, SIO module is greyed out in my TConf tool on CCSv5 (BIOS 5.42.0.07).
I could try the pipe, but I don't know how to add a blocking mechanism without a semaphore. I'm stuck again.