Re: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency
From: Robin Murphy
Date: Mon May 11 2015 - 13:24:18 EST
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.
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