Messages in this thread | | | Date | Sat, 30 Jun 2007 08:54:29 +0100 | From | Russell King <> | Subject | Re: [PATCH] Containment measures for slab objects on scatter gather lists |
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On Sat, Jun 30, 2007 at 12:11:38AM +0100, Alan Cox wrote: > > DMA to or from memory should be done via the DMA mapping API. If we're > > DMAing to/from a limited range within a page, either we should be using > > dma_map_single(), or dma_map_page() with an appropriate offset and size. > > If those ranges overlap a cache line then the dma mapping API will not > save your backside.
There's nothing much that the DMA API can do though. Consider DMA to a result buffer which is, eg, only 16 bytes in size. So you get passed a size of '16' to the DMA API. What should you do at this point? BUG()?
What if you have 64 or 128 byte cache lines?
> > sizes, but they do happen. We handle this on ARM by writing back > > the overlapped lines and invalidating the rest before the DMA operation > > commences, and hope that the overlapped lines aren't touched for the > > duration of the DMA.) > > The combination of "hope" and "DMA" isn't a good one for stable system > design. In this situation we should be waving large red flags
I agree.
However, I don't think this is an issue for the DMA API to handle; it's something that driver authors need to be aware of. If they wish to do a DMA to a kmalloc'd buffer or even a page, we could require that offsets and sizes are cache line aligned.
However, remember that turning on slab debugging turns off cache line alignment, so imposing such a requirement implies that the slab debugging will break DMA, or driver authors also have to be aware of that and do their own alignment internally, *or* we provide an allocator which does unconditionally align.
-- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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