On Wednesday 29 April 2015 14:57:10 Suthikulpanit, Suravee wrote: > 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. > My guess is that this section was included for Windows Phone, which runs on embedded SoCs that usually have noncoherent DMA in a particular way. Linux however only uses ACPI for servers, so that case does not happen. I guess it would be reasonable to add a run-time warning here if you try to do DMA on a device that does not have CCA set, and you should probably set the DMA mask to 0 in that case as well. Note that there are lots of ways in which you could have noncoherent DMA: the default on ARM32 is that it requires uncached access or explicit cache flushes, but it's also possible to have an SMP system where a device is only coherent with some of the CPUs and requires explicit synchronization (not flushes) otherwise. In a multi-level cache hierarchy, there could be all sorts of combinations of flushes and syncs you would need to do. With DT, we handle this using SoC-specific overrides for platforms that are noncoherent in funny ways, see http://lxr.free-electrons.com/source/arch/arm/mach-mvebu/coherency.c?v=3.18#L263 for instance. If we just disallow DMA to devices that are marked with _CCA=0 in ACPI, we can avoid this case, or discuss it by the time someone has hardware that wants it, and then make a more informed decision about it. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
