On Tue, Jun 08, 2021 at 09:20:29AM +0800, Jason Wang wrote:

"

6.2.17 _CCA (Cache Coherency Attribute) The _CCA object returns whether or
not a bus-master device supports hardware managed cache coherency. Expected
values are 0 to indicate it is not supported, and 1 to indicate that it is
supported. All other values are reserved.

...

On Intel platforms, if the _CCA object is not supplied, the OSPM will assume
the devices are hardware cache coherent.

"

_CCA is mostly used on arm/arm64 platforms to figure out if a device
needs non-coherent DMA handling in the DMA API or not.  It is not
related to the NoSnoop TLPs that override the setting for an otherwise
coherent device.