On Fri, May 08, 2015 at 04:08:53PM +0200, Arnd Bergmann wrote: > On Friday 01 May 2015 12:06:44 Catalin Marinas wrote: > > > 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. > > > > I don't think we should disallow DMA to devices with _CCA == 0 (only to > > those that don't have a _CCA property at all) as long as _CCA == 0 has > > clear semantics like only architected cache maintenance required (and > > that's what the ARMv8 ARM requires from compliant system caches). > > Even if we exclude all cases in which the behavior may be unexpected, > there is still the other point I raised initially: > > what would that be good for? > > Can you think of a case where a server system has a reason to use > a device in noncoherent mode? I think it's more likely to be a case > where a device got misconfigured accidentally by the firmware, and > we're better off warning about that in the kernel than trying to prepare > for an unknown hardware that might use an obscure feature of the spec. Maybe some of the people involved in arm64 servers can give a better answer, I'm not familiar with their hardware (plans). I would expect most DMA-capable devices to be cache coherent. However, for (system) performance reasons, some of them could be configured as non-coherent. An example, though unlikely on servers, is a display device continuously accessing a framebuffer. You may not want to overload the coherent interconnect. -- Catalin -- 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
