[V4 PATCH 1/6] ACPI / scan: Parse _CCA and setup device coherency
suravee.suthikulpanit at amd.com
Wed May 20 04:52:03 PDT 2015
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.
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