Am 16.01.19 um 01:33 schrieb Benjamin Herrenschmidt: > On Tue, 2019-01-15 at 22:31 +1100, Michael Ellerman wrote: >>>> As far as I know Power doesn't really supports un-cached memory at all, >>>> except for a very very old and odd configuration with AGP. >>> Hopefully Michael/Ben can elaborate here, but I was under the (possibly >>> mistaken) impression that mismatched attributes could cause a machine-check >>> on Power. >> That's what I've always been told, but I can't actually find where it's >> documented, I'll keep searching. >> >> But you're right that mixing cached / uncached is not really supported, >> and probably results in a machine check or worse. > .. or worse :) It could checkstop. Not sure if that would be so bad, it would at least give us a clear indicator that something is wrong instead of silently corrupting data. > It's also my understanding that on ARM v7 and above, it's technically > forbidden to map the same physical page with both cached and non-cached > mappings, since the cached one could prefetch (or speculatively load), > thus creating collisions and inconsistencies. Am I wrong here ? No, but you answer the wrong question. See we don't want to have different mappings of cached and non-cached on the CPU, but rather want to know if a snooped DMA from the PCIe counts as cached access as well. As far as I know on x86 it doesn't, so when you have an un-cached page you can still access it with a snooping DMA read/write operation and don't cause trouble. > The old hack of using non-cached mapping to avoid snoop cost in AGP and > others is just that ... an ugly and horrible hacks that should have > never eventuated, when the search for performance pushes HW people into > utter insanity :) Well I agree that un-cached system memory makes things much more complicated for a questionable gain. But fact is we now have to deal with the mess, so no point in complaining about it to much :) Cheers, Christian. > > Cheers, > Ben. > > > _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
