I have problems on chipset SiS 650/M650/651/740. When I try to log into X I receive this error:

Fatal server error:
xf86 MapVidMem: could not mmap framebuffer (0x00000000, 0x2000) (Invalid argument)

Booting the system with "no framebuffer" option and using vesa mode it works well but it doesn't the same using sis driver and therefore doesn't work neither if I boot with framebuffer.