The SGMII SerDes of the 3041 can operate at either 1.25G or 2.5G. Is there a register to configure this or it just depends on the clock multipliers of the SerDes PLL?
As long as you select the RCW settings for SRDS_PRTCL and the DIV and RATIO settings to select the clock speed, the SerDes registers will default to the correct value based on those RCW settings.
Does P3041 SerDes in XAUI mode support 10GBase-CX4 (IEEE802.3 clause54)? In other words, are SerDes XAUI receivers and transmitters capable of communication over a 15 meter 10GBase-CX4 cable?
P3041 is not designed for such long distances. An appropriate 10GBASE-CX4 PHY is needed to support such distances.