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

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Message-ID: <54B94674.10206@amd.com>
Date:	Fri, 16 Jan 2015 11:12:20 -0600
From:	Tom Lendacky <thomas.lendacky@....com>
To:	Will Deacon <will.deacon@....com>, Arnd Bergmann <arnd@...db.de>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <Mark.Rutland@....com>,
	linaro-acpi <linaro-acpi@...ts.linaro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Yijing Wang <wangyijing@...wei.com>,
	Rob Herring <robh@...nel.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Timur Tabi <timur@...eaurora.org>,
	ACPI Devel Mailing List <linux-acpi@...r.kernel.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@....com>,
	"phoenix.liyi@...wei.com" <phoenix.liyi@...wei.com>,
	Robert Richter <rric@...nel.org>,
	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"jcm@...hat.com" <jcm@...hat.com>, Mark Brown <broonie@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"graeme.gregory@...aro.org" <graeme.gregory@...aro.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"hanjun.guo@...aro.org" <hanjun.guo@...aro.org>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	"Olof Johansson" <olof@...om.net>
Subject: Re: [PATCH v7 00/17] Introduce ACPI for ARM64 based on ACPI 5.1

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
>
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