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 thedevice
* 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 _CCAobject.
** In both cases, we will default to dmanon-coherent.
*/cca = 0;} else {/** If architecture does not specify thatdevice must
* specify ACPI _CCA (e.g. x86), we defaultto 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?
ArndActually, 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