The control of the IDE bus and the UltraDMA is managed by the southbridge of the SIS745 chipset. The particularity of this chipset is that it groups together the northbridge and the southbrigde in the same chipset.
The IDE controller in the southbridge is based on the SIS5513 chipset family. So Linux uses the sis5513 driver to control it.
For more informations, see the /ide/pci/sis5513.c file.
Unfortunately, it seems that there are some problems to work with the UltraDMA 100. The reason is that the IDE driver uses the ID of the northbrigde to determine the type of the controller.
The 2.4.21 kernel seems to fix the problem.
You can get the latest kernel 2.4.x from http://www.linuxhq.com/kernel/v2.4/index.html.
In the kernel configuration menu (# make xconfig).
Say Y to "Use DMA by default when available".
Say Y to "SiS5513 chipset support" (CONFIG_BLK_DEV_SIS5513 variable).
And compile this new version of your kernel.