Re: [Linaro-acpi] [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.