Linear indexing apparatus with selective porting

A linear indexing tool is operably supportable on a tubing string in a subterranean wellbore and has a tubular housing structure within which a tubular mandrel is coaxially and slidably disposed. An indexing structure is operable by tubing pressure to sequentially drive the mandrel through a plurality of axial travel increments in response to a corresponding successive plurality of tubing pressure forces exerted on the indexing structure. Pressure diversion apparatus is operative to permit flow of pressurized fluid from within the housing structure to an external, pressure-actuatable device, such as a packer, only after the mandrel has been driven through at least two of its axial travel increments. Accordingly, the internal tubing pressure may be elevated to a test level thereof, thereby driving the mandrel through its first axial travel increment, without actuating the external device. A subsequent tubing pressure reduction and re-elevation may then be used to index the mandrel through another axial travel stroke to cause the internal tubing pressure to be operably diverted to the external device.