Re: [Linaro-acpi] [PATCH v7 00/17] Introduce ACPI for ARM64 based on ACPI 5.1

[Arnd Bergmann]

On Friday 16 January 2015 16:40:28 Arnd Bergmann wrote:
> On Friday 16 January 2015 15:33:20 Will Deacon wrote:
> > On Fri, Jan 16, 2015 at 03:14:13PM +0000, Arnd Bergmann wrote:
> > > On Friday 16 January 2015 14:55:45 Will Deacon wrote:
> > > > On Fri, Jan 16, 2015 at 02:45:30PM +0000, Tom Lendacky wrote:
> > > > > 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.

To clarify: I don't think we should just ignore _CCA in Linux, but instead
see if it lists the device as coherent and warn loudly if it's configured
as noncoherent, then set the dma-mask pointer to NULL to prevent DMA from
being started on it.

	Arnd
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