Problem with atomic accesses in pstore on some ARM CPUs

[Guenter Roeck]

On Tue, Aug 16, 2016 at 6:21 AM, Will Deacon <will.deacon at arm.com> wrote:
> On Tue, Aug 16, 2016 at 06:14:53AM -0700, Guenter Roeck wrote:
>> On Tue, Aug 16, 2016 at 3:32 AM, Robin Murphy <robin.murphy at arm.com> wrote:
>> > On 16/08/16 00:19, Guenter Roeck wrote:
>> >> we are having a problem with atomic accesses in pstore on some ARM
>> >> CPUs (specifically rk3288 and rk3399). With those chips, atomic
>> >> accesses fail with both pgprot_noncached and pgprot_writecombine
>> >> memory. Atomic accesses do work when selecting PAGE_KERNEL protection.
>> >
>> > What's the pstore backed by? I'm guessing it's not normal DRAM.
>> >
>>
>> it is normal DRAM.
>
> In which case, why does it need to be mapped with weird attributes?
> Is there an alias in the linear map you can use?
>

I don't really _want_ to do anything besides using pstore as-is, or,
in other words, to have the upstream kernel work with the affected
systems.

The current pstore code runs the following code for memory with
pfn_valid() = true.

        if (memtype)
                prot = pgprot_noncached(PAGE_KERNEL);
        else
                prot = pgprot_writecombine(PAGE_KERNEL);
        ...
        vaddr = vmap(pages, page_count, VM_MAP, prot);

It then uses the memory pointed to by vaddr for atomic operations.

In my case, both protection options don't work. Everything works fine
(or at least doesn't create an exception) if I use
        vaddr = vmap(pages, page_count, VM_MAP, PAGE_KERNEL);
instead.

Thanks,
Guenter