Looking at the Lantiq xDSL source code in Openwrt/LEDE we can see the underlying hardware architecture. There are three different processors involved.
The first in the main SoC cpu, generally a MIPS core. This has the main linux system running with drivers for the xDSL modem and datapath.
The second in the xDSL modem processor, an ARC. This is most often seen embedded in ASICs as it is supplied as IP from Synopsys. It is also used by Intel as a embedded management controller in their chipsets.
The third processor is the PPE, a proprietary Lantiq solution. This is responsible for data formatting and transfer.