On Mon, 2012-07-23 at 11:17 -0500, Scott Wood wrote: > > This is wrong. > > How so? > > > Don't you have an iommu do deal with those devices anyway ? > > Yes, but we don't yet have DMA API support for it, it would lower > performance because we'd have to use a lot of subwindows which are > poorly cached (and even then we wouldn't be able to map more than 256 > pages at once on a given device), and the IOMMU may not be available at > all if we're being virtualized.
Ugh ? You mean some designers need to be fired urgently and wasted everybody's time implementing an unusable iommu ? Nice one ... > > What about swiotlb ? > > That doesn't help with alloc_coherent(). Somewhat... you can use the pool, but it sucks. > > If you *really* need to honor 32 (or 31 even) bit DMAs, > > 31-bit is to accommodate PCI, which has PEXCSRBAR that must live under 4 > GiB and can't be disabled. > > > what you -may- want to do is create a ZONE_DMA32 like other architectures, > > do not > > hijack the historical ZONE_DMA. > > Could you point me to somewhere that clearly defines what ZONE_DMA is to > be used for, such that this counts as hijacking (but using ZONE_DMA32 to > mean 31-bit wouldn't)? Habit and history. ZONE_DMA used to be about ISA DMA, doesn't apply to us, and in general ZONE_NORMAL alias to it iirc. It's old stuff I haven't looked for a long time. However, my understanding is that what you are trying to solve is exactly what ZONE_DMA32 was created for. > The only arches I see using ZONE_DMA32 (x86 and > mips) also have a separate, more restrictive ZONE_DMA. And ? Who cares ? Drivers who know about a 32-bit limitations use GFP_DMA32, that's what is expected, don't mess around with ZONE_DMA. > PowerPC doesn't. > It uses ZONE_DMA to point to all of memory (except highmem on 32-bit) > -- how is that not hijacking, if this is? We can't have ZONE_DMA be > less restrictive than ZONE_DMA32, because the fallback rules are > hardcoded the other way around in generic code. > > The exact threshold for ZONE_DMA could be made platform-configurable. > > > But even then, I'm dubious this is really needed. > > We'd like our drivers to stop crashing with more than 4GiB of RAM on 64-bit. Fix your HW :-) Cheers, Ben. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
