On Friday 16 January 2015 15:33:20 Will Deacon wrote:

On Fri, Jan 16, 2015 at 03:14:13PM +0000, Arnd Bergmann wrote:

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On Friday 16 January 2015 14:55:45 Will Deacon wrote:

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On Fri, Jan 16, 2015 at 02:45:30PM +0000, Tom Lendacky wrote:

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I have tested ACPI-enablement patches for the amd-xgbe/amd-xgbe-phy
drivers that I'm about to submit upstream with the V7 patch series
on the AMD Seattle server platform. There does not appear to be support
for the _CCA attribute in this patch series. The amd-xgbe driver will
setup the device domain and cache attributes based on the presence of
this attribute, but it requires the arch support to assign the proper
DMA operations in order for it to all work correctly.

Overriding the _CCA attribute in the driver, I was able to successfully
test the driver and this patch series.

Hopefully this will all be addressed when the IORT parts of ACPI have
settled down (the current proposal allows for these attributes to be
described as well as their interaction with things like IOMMUs).

In the meantime, are you falling back to non-coherent DMA? If so, what
attributes have you settled on? We need to be really careful not to
corrupt data during cache invalidatation when mapping a non-coherent
buffer for the CPU.

I think in case of ACPI we should use cache-coherent as the default,
as this is what all servers will use for DMA masters.

I don't agree. The dma-coherent we have for device-tree isn't nearly
expressive enough for the kind of things we want to describe and there's
no reason to make the same mistake in ACPI, especially as it *is* being
addressed by IORT. If we run with _CCA, then we're going to be stuck
supporting something that isn't fit for purpose and which will likely be
abused to describe both fixed features of the system and software
configuration preferences. It also opens up a can of worms if we have to
support a mixture of _CCA and IORT in the future.

Or are you suggesting that we ignore _CCA and just assume cache-coherency?
In that case, how do we support systems that aren't cache coherent, where
not being cache coherent includes devices that require either device or
IOMMU configuration to enable cacheable transactions?

I was thinking we'd ignore _CCA because as you say a simple on/off flag
would not be enough to describe what we have to do for noncoherent
devices. I can't think of any reason why a server hardware would include
noncoherent devices, so if they are configurable they should be configured
into coherent mode by the firmware.

	Arnd