From: Arnd Bergmann <arnd@arndb.de>
To: linaro-acpi@lists.linaro.org
Cc: Suravee Suthikulanit <suravee.suthikulpanit@amd.com>,
        Catalin Marinas <catalin.marinas@arm.com>, herbert@gondor.apana.org.au,
        rjw@rjwysocki.net, will.deacon@arm.com, linux-kernel@vger.kernel.org,
        linux-acpi@vger.kernel.org, linux-crypto@vger.kernel.org,
        netdev@vger.kernel.org, bhelgaas@google.com, davem@davemloft.net,
        linux-arm-kernel@lists.infradead.org, lenb@kernel.org
Subject: Re: [Linaro-acpi] [V4 PATCH 1/6] ACPI / scan: Parse _CCA and setup device coherency
Date: Wed, 20 May 2015 14:04:02 +0200
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On Wednesday 20 May 2015 06:52:03 Suravee Suthikulanit wrote:
> On 5/20/2015 5:01 AM, Catalin Marinas wrote:
> > 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.
> 
> Ok. That seems to be what Arnd would prefer as well.  Let's just leave 
> the support for _CCA=0 out until it is needed then.
> 

Yes, that would be best (as I said repeatedly ;-) )

	Arnd
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