Re: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency

[Robin Murphy <robin.murphy@arm.com>]

On 11/05/15 18:10, Catalin Marinas wrote:
> On Fri, May 08, 2015 at 04:08:53PM +0200, Arnd Bergmann wrote:
>> On Friday 01 May 2015 12:06:44 Catalin Marinas wrote:
>>>> If we just disallow DMA to devices that are marked with _CCA=0
>>>> in ACPI, we can avoid this case, or discuss it by the time someone has hardware
>>>> that wants it, and then make a more informed decision about it.
>>>
>>> I don't think we should disallow DMA to devices with _CCA == 0 (only to
>>> those that don't have a _CCA property at all) as long as _CCA == 0 has
>>> clear semantics like only architected cache maintenance required (and
>>> that's what the ARMv8 ARM requires from compliant system caches).
>>
>> Even if we exclude all cases in which the behavior may be unexpected,
>> there is still the other point I raised initially:
>>
>>               what would that be good for?
>>
>> Can you think of a case where a server system has a reason to use
>> a device in noncoherent mode? I think it's more likely to be a case
>> where a device got misconfigured accidentally by the firmware, and
>> we're better off warning about that in the kernel than trying to prepare
>> for an unknown hardware that might use an obscure feature of the spec.
>
> Maybe some of the people involved in arm64 servers can give a better
> answer, I'm not familiar with their hardware (plans).
>
> I would expect most DMA-capable devices to be cache coherent. However,
> for (system) performance reasons, some of them could be configured as
> non-coherent. An example, though unlikely on servers, is a display
> device continuously accessing a framebuffer. You may not want to
> overload the coherent interconnect.

FWIW, I've also had much the same argument put to me for IOMMUs, i.e. 
they want to make the page table walk interface non-coherent because 
they'd rather pay the cost of flushing the page tables once to save a 
few extra cycles of latency for cache snooping on every TLB miss.

Robin.