lkml.org 
[lkml]   [2015]   [Jan]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms
    From
    On Thu, Jan 29, 2015 at 5:31 PM, Russell King - ARM Linux
    <linux@arm.linux.org.uk> 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@arm.linux.org.uk> 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.


    \
     
     \ /
      Last update: 2015-01-30 00:41    [W:4.261 / U:0.108 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site