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

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Date:	Wed, 29 Apr 2015 16:03:19 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
	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 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
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