__iomem is a kernel cookie used by Sparse, a semantic checker used by the kernel to find possible coding faults. To take advantage of the features offered by Sparse, it should be enabled at kernel compile time; if not, __iomem cookie will be ignored anyway.
The C=1 in the command line will enable Sparse for you, but Sparse should be installed first on your system:
sudo apt-get install sparse
For example, when building a module, use:
make -C $KPATH M=$PWD C=1 modules
Alternatively, if the makefile is well-written, just type:
make C=1
The following shows how __iomem is defined in the kernel:
#define __iomem __attribute__((noderef, address_space(2)))
It protects us from faulty drivers performing I/O memory access. Adding the ...
