Full RAM size not recognized by linux kernel after installing RT patch

Question

I use 32-bit PowerPC with the Linux 4.1.8 kernel, and just applied the full RT patch which turned off the CONFIG_HIGHMEM configuration in menuconfig.

Following that my RAM size halved. The kernel recognizes only 766 MB out of 1.5 GB.

I saw there is a patch for restoring CONFIG_HIGHMEM configuration while still applying the RT patch but its working for ARM only.

How can I change the settings in order the kernel to recognize the whole RAM without using high memory area? (Maybe create more zone_normal? More nodes? Or just extend the low memory area?)

Thanks!