On Friday 08 July 2011, Russell King - ARM Linux wrote: > On Tue, Jul 05, 2011 at 03:58:39PM +0200, Arnd Bergmann wrote: > > > If I'm reading your "ARM: DMA: steal memory for DMA coherent mappings" > > correctly, the idea is to have a per-platform compile-time amount > > of memory that is reserved purely for coherent allocations and > > taking out of the buddy allocator, right? > > Yes, because every time I've looked at taking out memory mappings in > the first level page tables, it's always been a major issue. > > We have a method where we can remove first level mappings on > uniprocessor systems in the ioremap code just fine - we use that so > that systems can setup section and supersection mappings. They can > tear them down as well - and we update other tasks L1 page tables > when they get switched in. > > This, however, doesn't work on SMP, because if you have a DMA allocation > (which is permitted from IRQ context) you must have some way of removing > the L1 page table entries from all CPUs TLBs and the page tables currently > in use and any future page tables which those CPUs may switch to. Ah, interesting. So there is no tlb flush broadcast operation and it always goes through IPI? > So, in a SMP system, there is no safe way to remove L1 page table entries > from IRQ context. That means if memory is mapped for the buddy allocators > using L1 page table entries, then it is fixed for that application on a > SMP system. Ok. Can we limit GFP_ATOMIC to memory that doesn't need to be remapped then? I guess we can assume that there is no regression if we just skip the dma_alloc_contiguous step in dma_alloc_coherent for any atomic callers and immediately fall back to the regular allocator. Unfortunately, this still means we have to keep both methods. I was hoping that with CMA doing dynamic remapping there would be no need for keeping a significant number of pages reserved for this. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
