Creating kernel mappings for memory initially marked with bootmem NOMAP?

[Ard Biesheuvel]

> On 8 Mar 2017, at 20:03, Florian Fainelli <f.fainelli at gmail.com> wrote:
> 
> Hi,
> 
> On our platforms (brcmstb) we have an use case where we boot with some
> (a lot actually) memory carved out and marked initially with bootmem
> NOMAP in order for this memory not to be mapped in the kernel's linear
> mapping.
> 
> Now, we have some peripherals that want large chunks of physically and
> virtually contiguous memory that belong to these memblock NOMAP ranges.
> I have no problems using mmap() against this memory, because the kernel
> will do what is necessary for a process to map it for me. The struggle
> is for a kernel driver which specifies a range of physical memory and
> size, and expects a virtually contiguous mapping in return (not using
> DMA-API, because reasons).
> 
> Essentially the problem is that there are no PTEs created for these
> memory regions (and pfn_valid() returns 0, since this is NOMAP memory),
> so I have been playing with __add_pages() from the memory hotplug code
> in an attempt to get proper page references to this memory, but I am
> clearly missing something.
> 
> Yes I know it's a terrible idea, but what if I wanted to get that working?
> 

Did you try memremap?