On Thu, Jan 29, 2015 at 5:31 PM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > On Thu, Jan 29, 2015 at 05:18:33PM -0500, Rob Clark wrote: >> On Thu, Jan 29, 2015 at 2:26 PM, Russell King - ARM Linux >> <linux@xxxxxxxxxxxxxxxx> wrote: >> > Now, if we're going to do the "more clever" thing you mention above, >> > that rather negates the point of this two-part patch set, which is to >> > provide the union of the DMA capabilities of all users. A union in >> > that case is no longer sane as we'd be tailoring the SG lists to each >> > user. >> >> It doesn't really negate.. a different sg list representing the same >> physical memory cannot suddenly make the buffer physically contiguous >> (from the perspective of memory).. >> >> (unless we are not on the same page here, so to speak) > > If we are really only interested in the "physically contiguous" vs > "scattered" differentiation, why can't this be just a simple flag? I'd be fine with that.. I was trying to make it a bit less of a point solution, but maybe trying to be too generic is not worth it.. There is apparently some hw which has iommu's but small # of tlb entries, and would prefer partially contiguous buffers. But that isn't a hard constraint, and maybe shouldn't be solved w/ max_segment_count. And I'm not sure how common that is. > I think I know where you're coming from on that distinction - most > GPUs can cope with their buffers being discontiguous in memory, but > scanout and capture hardware tends to need contiguous buffers. > > My guess is that you're looking for some way that a GPU driver could > allocate a buffer, which can then be imported into the scanout > hardware - and when it is, the underlying backing store is converted > to a contiguous buffer. Is that the usage scenario you're thinking > of? Pretty much.. and maybe a few slight permutations on that involving cameras / video codecs / etc. But the really-really common case is gpu (with mmu/iommu) + display (without). Just solving this problem would be a really good first step. BR, -R > > -- > FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up > according to speedtest.net. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>