The following is based on the assumption that you have done a complete build. See https://github.com/TritonDataCenter/smartos-live#building-smartos.
cd illumos/usr/src
./tools/proto/root_i386-nd/opt/onbld/bin/bldenv ../../illumos.sh
cd uts/intel/<drivername>
dmake
use
git ls-files |grep <drivername>
and/or
git ls-files | grep '<drivername>/Makefile'
when locating file for a particular driver
When developing a new device driver the convention is to have:
common/io/<drivername>
for codeintel/<drivername>/Makefile
for makefile on x86sparc/<drivername>/Makefile
for makefile on SPARC