linux-kernel - Re: [V4 PATCH 1/6] ACPI / scan: Parse

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Message-ID: <20150520100122.GD25313@e104818-lin.cambridge.arm.com>
Date:	Wed, 20 May 2015 11:01:22 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Cc:	rjw@...ysocki.net, lenb@...nel.org, will.deacon@....com,
	bhelgaas@...gle.com, thomas.lendacky@....com,
	herbert@...dor.apana.org.au, davem@...emloft.net, arnd@...db.de,
	al.stone@...aro.org, linaro-acpi@...ts.linaro.org,
	netdev@...r.kernel.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,
	linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org
Subject: Re: [V4 PATCH 1/6] ACPI / scan: Parse _CCA and setup device coherency

On Fri, May 15, 2015 at 04:23:09PM -0500, Suravee Suthikulpanit wrote:
> +static inline bool acpi_dma_is_supported(struct acpi_device *adev)
> +{
> +	/**
> +	 * Currently, we mainly support _CCA=1 (i.e. is_coherent=1)
> +	 * This should be equivalent to specifyig dma-coherent for
> +	 * a device in OF.
> +	 *
> +	 * For the case when _CCA=0 (i.e. is_coherent=0 && cca_seen=1),
> +	 * There are two approaches:
> +	 * 1. Do not support and disable DMA.
> +	 * 2. Support but rely on arch-specific cache maintenance for
> +	 * non-coherence DMA operations. ARM64 is one example.
> +	 *
> +	 * For the case when _CCA is missing (i.e. cca_seen=0) but
> +	 * platform specifies ACPI_CCA_REQUIRED, we do not support DMA,
> +	 * and fallback to arch-specific default handling.
> +	 *
> +	 * See acpi_init_coherency() for more info.
> +	 */
> +	return adev && (adev->flags.is_coherent ||
> +			(adev->flags.cca_seen && IS_ENABLED(CONFIG_ARM64)));
> +}

I don't particularly like the check for CONFIG_ARM64 here but I
understand why it was added (I had the wrong impression that x86 can
cope with _CCA = 0).

Alternatively, we could leave it out (together with cca_seen) until
someone comes forward with a real use-case for _CCA = 0 on arm64. One
platform I'm aware of is Juno but even though it boot with ACPI, I
wouldn't call it a server platform.

-- 
Catalin
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