Creating kernel mappings for memory initially marked with bootmem NOMAP?

[f.fainelli@gmail.com (Florian Fainelli)]

On 03/08/2017 11:14 AM, Ard Biesheuvel wrote:
> 
>> On 8 Mar 2017, at 20:03, Florian Fainelli <f.fainelli@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?

Not yet, because this is done on 4.1 at the moment, but I will
definitively give this a try, thanks a lot!

Side note: on a kernel that does not have memremap() (such as 4.1),
would not an ioremap_cache() on the physical range marked as NOMAP
result in the same behavior anyway? ioremap() won't catch the fact that
we are mapping RAM, since this is NOMAP, pfn_valid() returns 0.

My understanding of the pfn_valid() check for ioremap() is to avoid
mapping the same DRAM location twice with potentially conflicting
attributes, but if it has not been mapped at all, as is the case with
NOMAP, does not that get me the same results?

Thanks!
-- 
Florian