On 07/23/2012 01:06 AM, Benjamin Herrenschmidt wrote: > On Fri, 2012-07-20 at 20:21 +0800, Shaohui Xie wrote: >> PowerPC platform only supports ZONE_DMA zone for 64bit kernel, so all the >> memory will be put into this zone. If the memory size is greater than >> the device's DMA capability and device uses dma_alloc_coherent to allocate >> memory, it will get an address which is over the device's DMA addressing, >> the device will fail. >> >> So we split the memory to two zones by adding a zone ZONE_NORMAL, since >> we already allocate PCICSRBAR/PEXCSRBAR right below the 4G boundary (if the >> lowest PCI address is above 4G), so we constrain the DMA zone ZONE_DMA >> to 2GB, also, we clear the flag __GFP_DMA and set it only if the device's >> dma_mask < total memory size. By doing this, devices which cannot DMA all >> the memory will be limited to ZONE_DMA, but devices which can DMA all the >> memory will not be affected by this limitation. > > 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. > What about swiotlb ? That doesn't help with alloc_coherent(). > 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)? The only arches I see using ZONE_DMA32 (x86 and mips) also have a separate, more restrictive 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. -Scott _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
