High mem in arm Architecture

High memory (highmem) is used when the size of physical memory approaches or exceeds the maximum size of virtual memory. The traditional split for architectures using this approach is 3:1, 3GiB for userspace and the top 1GiB for kernel space. This means kernel can at most map 1 GiB of physical memory.

In mobile devices average physical memory present is around 1 GiB. So kernel can map that much amount of memory.

Is there need of highmem in devices with 1GiB of physical memory?

How much physical memory we can address using highmem config option ?

Thanks

Answer

Yes, hignmem is required, some of the 1GiB address space is required for memory mapped IO and non-linear mappings of physical RAM.