With USE_EXPAND in portage-2.1* you get the following output for for example VIDEO_CARD and INPUT_DEVICES: betelgeuse@pena /usr/portage/media-sound/alsa-driver $ emerge -pv xorg-x11 These are the packages that would be merged, in order: Calculating dependencies... done! [ebuild R ] x11-base/xorg-x11-7.0-r1 USE="-3dfx" INPUT_DEVICES="keyboard mouse -acecad -aiptek -calcomp -citron -digitaledge -dmc -dynapro -elo2300 -elographics -evdev -fpit -hyperpen -jamstudio -joystick -magellan -magictouch -microtouch -mutouch -palmax -penmount -spaceorb -summa -synaptics -tek4957 -ur98 -vmmouse -void -wacom" VIDEO_CARDS="nvidia -apm -ark -ati -chips -cirrus -cyrix -dummy -fbdev -fglrx -glint -i128 -i740 -i810 -imstt -mga -neomagic -nsc -nv -rendition -s3 -s3virge -savage -siliconmotion -sis -sisusb -tdfx -tga -trident -tseng -v4l -vesa -vga -via -vmware -voodoo" 0 kB Total size of downloads: 0 kB It would be nice to have the same listing for ALSA_CARDS. This can be done by adding the drivers in the form alsa_cards_driver to IUSE and ALSA_CARDS to USE_EXPAND in profiles/make.defaults.
Not feasible right now, as ALSA_CARDS is not used as a collection of useflags, but it's used as a variable by itself.