Re: [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency

Suravee Suthikulpanit
2015-04-29T14:46:03Z

On 04/29/2015 09:03 AM, Arnd Bergmann wrote:
> On Wednesday 29 April 2015 08:44:09 Suravee Suthikulpanit wrote:
>> + device->flags.cca_seen = 1;
>> + } else if (IS_ENABLED(CONFIG_ACPI_MUST_HAVE_CCA)) {
>> + /*
>> + * Architecture has specified that if the device
>> + * can do DMA, it must have ACPI _CCA object.
>> + * Here, there could be two cases:
>> + * 1. Not DMA-able device.
>> + * 2. DMA-able device, but missing _CCA object.
>> + *
>> + * In both cases, we will default to dma non-coherent.
>> + */
>> + cca = 0;
>> + } else {
>> + /*
>> + * If architecture does not specify that device must
>> + * specify ACPI _CCA (e.g. x86), we default to use
>> + * dma coherent.
>> + */
>> + cca = 1;
>> + }
>>
>
> What does it mean here if a device does DMA but is not coherent? Do you
> have an example of a server that needs this?
>
> Can we please make the default for ARM64 cca=1 as well?
>
> Arnd
>

Actually, I am trying to implement the logic for when missing _CCA to be
consistent with the behavior when the devicetree entry does not specify
"dma-coherent" property. IIUC, in such case, Linux will default to using
non-coherent DMA.

Thanks,

Suravee