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

[Arnd Bergmann <arnd@arndb.de>]

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