The following specs will be based on Broadcom's adsl/atm drivers for the bcm63xx line of SoCs for the 18.104.22.168 and 22.214.171.124 kernels. Specs for the bcm6348 are in progress, while bcm6338/6345/6358 are in queue. Bcm6368 seems to use a different architecture, so this will be tackled last.
These specs are incomplete.
BCM63XX SoC Specification
Overview over the different SoCs (mostly differences in their wiring/features).
Backplane addresses and cores (only those relevant to ADSL/ATM)
Exchanging Messages with the ADSL PHY
Access to the UBUS attached Wifi