Kernel support seems to be:
fs/btrfs/super.c: {Opt_discard, "discard"}, fs/ext4/super.c: {Opt_discard, "discard"}, fs/fat/inode.c: {Opt_discard, "discard"}, fs/gfs2/super.c: {Opt_discard, "discard"}, fs/nilfs2/super.c: {Opt_discard, "discard"},
Kernel support seems to be:
fs/btrfs/super.c: {Opt_discard, "discard"},
fs/ext4/super.c: {Opt_discard, "discard"},
fs/fat/inode.c: {Opt_discard, "discard"},
fs/gfs2/super.c: {Opt_discard, "discard"},
fs/nilfs2/super.c: {Opt_discard, "discard"},