On Thu, Jan 29, 2015 at 5:31 PM, Russell King - ARM Linux
[off-list ref] wrote:

On Thu, Jan 29, 2015 at 05:18:33PM -0500, Rob Clark wrote:

quoted

On Thu, Jan 29, 2015 at 2:26 PM, Russell King - ARM Linux
[off-list ref] wrote:

quoted

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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