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

Tom Lendacky
2015-01-16T17:12:20Z

On 01/16/2015 09:49 AM, Will Deacon wrote:
> On Fri, Jan 16, 2015 at 03:40:28PM +0000, 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.
>
> The on-board ethernet on Seattle requires the driver to program its AXI
> attributes, so configuring it to be a coherent master actually means
> "program the same cacheable AXI settings as you have on the CPU". That
> sounds like Linux should be doing it to me, but even if the firmware takes
> a guess at "normal cacheable WBRWA", it's not clear to me whether that
> register persists across things like adapter reset.
>
> Tom?

The registers that contain the AxDOMAIN and AxCACHE settings do not
persist across an adapter reset.

Tom

>
> There's also the situation where the firmware hasn't initialised the
> register and Linux realises this during probe. What should it do then?
>
> Will
>