CONFIG_U3_DART is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_U3_DART (not configurable)

modulename: dart_iommu.ko

configname: CONFIG_U3_DART

Linux/PowerPC Kernel Configuration
└─>Platform support
└─>U3_DART
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
source code:
arch/powerpc/sysdev/dart_iommu.c