Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

[Rob Clark <robdclark@gmail.com>]

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

>
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