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