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

From: Arnd Bergmann
Date: Thu Apr 30 2015 - 06:48:01 EST

On Thursday 30 April 2015 11:41:02 Will Deacon wrote:
> Hi Arnd,
> On Thu, Apr 30, 2015 at 09:23:59AM +0100, Arnd Bergmann wrote:
> > On Wednesday 29 April 2015 16:53:10 Suravee Suthikulpanit wrote:
> > > As for the case where _CCA=0, I think the ACPI driver should essentially
> > > communicate the information as HW is non-coherent as described in the
> > > spec, and should be calling arch_setup_dma_ops(dev, false). It is true
> > > that this in probably less-likely for the ARM64 server platforms.
> > > However, I would think that the ACPI driver should not be making such
> > > assumption.
> >
> > Can you add a description to the ACPI spec then to describe in detail what
> > "non-coherent" is supposed to mean, and which action the OS is supposed to
> > take when accessing data from device or CPU?
> You may be interested in the IORT ACPI companion spec here:
> On CCA, it says:
> `This value must match the value returned by the _CCA object defined in
> the DSDT for the device represented by this node. The attribute can take
> the following values:
> - 0x1: The device is fully coherent. No cache maintenance[1] is required for
> memory shared with the device which is mapped on CPUs as
> Inner Write-Back (IWB), Outer Write-back (OWB), and Inner
> shareable (ISH). In addition, during system initialization at cold
> boot, or after wakeup from low-power state, if the cache
> coherency requires an SMMU override or some specific device
> configuration, the platform firmware has to ensure that this has
> been done. Therefore the semantics represented by a value of
> 0x1 are always correct at the time of hand-off from firmware to
> OS.

Ok, this part absolutely makes sense.

> - 0x0: The device is not coherent. Therefore:
> * Cache maintenance is required for memory shared with the
> device that is mapped on CPUs as IWB-OWB-ISH.

This still seems insufficient. I guess this excludes having to
synchronize external bridges or write buffers, but it does not specify
what cache maintenance is required. Should there be an "outer-flush"?
Should the CPU cache be invalidated or flushed (or both), and do
we need to care about caches inside of the device or just inside of
the CPU?

> * No cache maintenance is required for memory shared with the
> device that is mapped on the CPU as device or Non-cacheable.
> All other values are reserved.
> [1] Note: Caching operations described in this document apply to the CPU
> caches and any other caches in the system where device memory accesses
> can hit.'
> This aside, the documented introduces some useful, related concepts such
> as CPM (coherent path to memory) and DACS (device attributes are cacheable
> and inner shareable) for describing different IO subsystems. It also has
> mechanisms to descibe ID repainting from PCI->SMMU->ITS.

Ah, good.

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