I'll go out on a limb and suggest 'kern.cam.da%d.max_tags" or
similar, as this will be useful for more than just the Adaptec chip.
OK, you guys are exceeding my knowledge :)
cam_sim_alloc() seems to be the fn that ahd_attach() calls
that sets this.
That just sets it on a per-controller basis. You could also control
it on a per device basis by modifying the code in cam_periph.c that
handles peripheral driver instances. Support for both should be
pretty easy to add and both have their uses.