[RFC PATCH 1/3] pstore-ram: use write-combine mappings

[Rob Herring]

On 04/15/2013 05:21 PM, Colin Cross wrote:
> On Wed, Apr 10, 2013 at 6:30 AM, Rob Herring <robherring2 at gmail.com> wrote:
>> On 04/09/2013 10:53 PM, Colin Cross wrote:
>>> On Tue, Apr 9, 2013 at 8:08 PM, Rob Herring <robherring2 at gmail.com> wrote:
>>>> From: Rob Herring <rob.herring at calxeda.com>
>>>>
>>>> Atomic operations are undefined behavior on ARM for device or strongly
>>>> ordered memory types. So use write-combine variants for mappings. This
>>>> corresponds to normal, non-cacheable memory on ARM. For many other
>>>> architectures, this change should not change the mapping type.
>>>
>>> This is going to make ramconsole less reliable.  A debugging printk
>>> followed by a __raw_writel that causes an immediate hard crash is
>>> likely to lose the last updates, including the most useful message, in
>>> the write buffers.
>>
>> It would have to be a write that hangs the bus. In my experience with
>> AXI, the bus doesn't actually hang until you hit max outstanding
>> transactions.
> 
> I've seen many cases where a single write to device memory in an
> unclocked slave will completely and instantly hang all cpus, and the
> next write will never happen.
> 
>> I think exclusive stores will limit the buffering, but that is probably
>> not architecturally guaranteed.
>>
>> I could put a wb() in at the end of persistent_ram_write.
>>
>>> Also, isn't this patch unnecessary after patch 3 in this set?
>>
>> It is still needed in the main memory case to be architecturally correct
>> to avoid multiple mappings of different memory types and exclusive
>> accesses to device memory. At least on an A9, it doesn't really seem to
>> matter. I could remove this for the ioremap case.
> 
> According to my reading of the latest ARM ARM (Issue C, section
> A3.5.7), and Catalin's excellent explanation
> (http://lists.linaro.org/pipermail/linaro-dev/2012-February/010239.html),
> it is no longer considered unpredictable to have both cached and
> non-cached mappings to the same memory, as long as you use proper
> cache maintenance between accessing the two mappings.
> 
> In pstore_ram the cached mapping will never be accessed (and we don't
> care about speculative accesses), so no cache maintenance is
> necessary.  I don't see any need for this patch, and I see plenty of
> possible problems.

Exclusive accesses still have further restrictions. From section 3.4.5:

• It is IMPLEMENTATION DEFINED whether LDREX and STREX operations can be
performed to a memory region
   with the Device or Strongly-ordered memory attribute. Unless the
implementation documentation explicitly
  states that LDREX and STREX operations to a memory region with the
Device or Strongly-ordered attribute are
 permitted, the effect of such operations is UNPREDICTABLE.


Given that it is implementation defined, I don't see how Linux can rely
on that behavior.

Rob