http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57907
--- Comment #1 from Richard Earnshaw <rearnsha at gcc dot gnu.org> --- You get the warning because Cortex-A15 implements a superset of ARM-v7a, due to the addition of the HW division instructions. This means that -march=armv7-a and -mcpu=cortex-a15 leads to an ambiguity as to what architectural features you want and hence the warning. It's not currently possible to specify the exact architecture variant that A15 supports in the command line; we're looking at fixing that in future.
