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

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Date:	Wed, 29 Apr 2015 09:45:43 -0500
From:	Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:	Arnd Bergmann <arnd@...db.de>,
	<linux-arm-kernel@...ts.infradead.org>
CC:	<rjw@...ysocki.net>, <lenb@...nel.org>, <catalin.marinas@....com>,
	<will.deacon@....com>, <al.stone@...aro.org>,
	<linaro-acpi@...ts.linaro.org>, <linux-kernel@...r.kernel.org>,
	<linux-acpi@...r.kernel.org>, <leo.duran@....com>,
	<hanjun.guo@...aro.org>, <msalter@...hat.com>,
	<grant.likely@...aro.org>
Subject: Re: [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency



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.

Thanks,

Suravee
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