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

Suthikulpanit, Suravee
2015-04-29T14:57:38Z

On 4/29/15, 09:47, "Arnd Bergmann" <arnd@arndb.de> wrote:

>On Wednesday 29 April 2015 09:45:43 Suravee Suthikulpanit wrote:
>> 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.
>
>Why?
>
> Arnd

Otherwise, it would seem inconsistent with what states in the ACPI spec:

CCA objects are only relevant for devices that can access CPU-visible
memory,
such as devices that are DMA capable. On ARM based systems, the _CCA
object
must be supplied all such devices. On Intel platforms, if the _CCA
object is
not supplied, the OSPM will assume the devices are hardware cache
coherent.

>From the statement above, I interpreted as if it is not present, it would
be non-coherent.

Suravee