linux-kernel - Re: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse

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Date:	Wed, 29 Apr 2015 14:57:10 +0000
From:	"Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
To:	Arnd Bergmann <arnd@...db.de>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"will.deacon@....com" <will.deacon@....com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"Charles Garcia-Tobin" <Charles.Garcia-Tobin@....com>
Subject: Re: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse _CCA and setup
 device coherency



On 4/29/15, 09:47, "Arnd Bergmann" <arnd@...db.de> wrote:

>On Wednesday 29 April 2015 09:45:43 Suravee Suthikulpanit wrote:
>> On 04/29/2015 09:03 AM, Arnd Bergmann wrote:
>> > On Wednesday 29 April 2015 08:44:09 Suravee Suthikulpanit wrote:
>> >> +                       device->flags.cca_seen = 1;
>> >> +               } else if (IS_ENABLED(CONFIG_ACPI_MUST_HAVE_CCA)) {
>> >> +                       /*
>> >> +                        * Architecture has specified that if the
>>device
>> >> +                        * can do DMA, it must have ACPI _CCA object.
>> >> +                        * Here, there could be two cases:
>> >> +                        *   1. Not DMA-able device.
>> >> +                        *   2. DMA-able device, but missing _CCA
>>object.
>> >> +                        *
>> >> +                        * In both cases, we will default to dma
>>non-coherent.
>> >> +                        */
>> >> +                       cca = 0;
>> >> +               } else {
>> >> +                       /*
>> >> +                        * If architecture does not specify that
>>device must
>> >> +                        * specify ACPI _CCA (e.g. x86), we default
>>to use
>> >> +                        * dma coherent.
>> >> +                        */
>> >> +                       cca = 1;
>> >> +               }
>> >>
>> >
>> > What does it mean here if a device does DMA but is not coherent? Do
>>you
>> > have an example of a server that needs this?
>> >
>> > Can we please make the default for ARM64 cca=1 as well?
>> >
>> >       Arnd
>> >
>> 
>> Actually, I am trying to implement the logic for when missing _CCA to
>>be 
>> consistent with the behavior when the devicetree entry does not specify
>> "dma-coherent" property. IIUC, in such case, Linux will default to
>>using 
>> non-coherent DMA.
>
>Why?
>
>	Arnd

Otherwise, it would seem inconsistent with what states in the ACPI spec:
 
  CCA objects are only relevant for devices that can access CPU-visible
memory,
  such as devices that are DMA capable. On ARM based systems, the _CCA
object 
  must be supplied all such devices. On Intel platforms, if the _CCA
object is 
  not supplied, the OSPM will assume the devices are hardware cache
coherent.

>From the statement above, I interpreted as if it is not present, it would
be non-coherent.

Suravee

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