r_texturemode accepts at least the following three variables:
GL_NEAREST_MIPMAP_NEAREST - Linear filter, no mipmap
GL_LINEAR_MIPMAP_NEAREST - Linear filter, nearest mipmap
GL_LINEAR_MIPMAP_LINEAR - Linear filter, linear mipmap
The first is obviously the fastest setting, but it's also very, very ugly/pixelated/fantastic. Whatever rocks your boat.
The second being the standard setting, bilinear filtering
The third being the high quality setting, trilinear filtering
You can set other modes too, but these three are the most common ones I have used over the years.