Date: Wed, 20 May 2015 11:01:22 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Cc: rjw@rjwysocki.net, lenb@kernel.org, will.deacon@arm.com,
        bhelgaas@google.com, thomas.lendacky@amd.com,
        herbert@gondor.apana.org.au, davem@davemloft.net, arnd@arndb.de,
        al.stone@linaro.org, linaro-acpi@lists.linaro.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-acpi@vger.kernel.org, leo.duran@amd.com, hanjun.guo@linaro.org,
        msalter@redhat.com, grant.likely@linaro.org,
        linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org
Subject: Re: [V4 PATCH 1/6] ACPI / scan: Parse _CCA and setup device coherency
Message-ID: <20150520100122.GD25313@e104818-lin.cambridge.arm.com>
References: <1431724994-21601-1-git-send-email-Suravee.Suthikulpanit@amd.com>
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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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