[RFC PATCH] ARM64: PCI: inherit root controller's dma-coherent

[Catalin Marinas]

On Thu, Nov 27, 2014 at 09:05:57AM +0000, Arnd Bergmann wrote:
> On Thursday 27 November 2014 02:39:59 Jon Masters wrote:
> > We're looking at this at the moment (in addition to having the ACPI specification
> > clarified to indicate that there is no safe default assumption, requiring that
> > DMA masters always indicate their coherency or otherwise via a _CCA whether true
> > or false). 
> 
> I think for arm64, this should be easy: since ACPI is only for servers, we can
> rely on the fact that the devices have cache-coherent DMA all the time. Let's
> not over-complicate the ACPI case with all the special hacks we need for embedded.

As usual, we need hw vendors to say what they expect/build here.

I think in the past (non-ARM) the assumption was that the DMA is
coherent on ACPI-capable systems unless otherwise stated but I've seen
discussions (as Jon mentioned above) that ACPI should always indicate
the coherency on ARM without any assumption.

Either way, I don't think it's a problem for the kernel. We just need to
change the default DMA ops to coherent when booting with ACPI (using
non-coherent ops for a coherent device is not safe as the CPU can
corrupt cache lines written by the device).

-- 
Catalin