[Intel-gfx] [RFC 1/7] drm/i915: Extract sg creation into a helper

[Chris Wilson]

On Thu, Oct 13, 2016 at 10:55:12AM +0100, Tvrtko Ursulin wrote:
> 
> On 13/10/2016 10:20, Chris Wilson wrote:
> >On Thu, Oct 13, 2016 at 10:03:58AM +0100, Tvrtko Ursulin wrote:
> >>From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>
> >>In order to reuse the same logic in several places in the driver,
> >>extract the logic which adds pages to the sg list and does the
> >>potential coalescing, into separate functions.
> >>
> >>Code wanting to build the sg table needs to do the following:
> >>
> >>1. Call i915_sg_create to create the state object for a given
> >>    maximum number of pages.
> >>2. Iterate using i915_sg_for_each_page
> >>3. Call i915_sg_add_page to add all the pages in order
> >>4. On success call i915_sg_complete, or on failure i915_sg_abort
> >>
> >>In this patch only i915_gem_object_get_pages_gtt gets converted
> >>to use the new functions.
> >Yup, much happier. Though can we just allocate the state on the stack,
> >it is the same as what we are previously using anyway. 40 bytes.
> 
> Thought about this also and had a slight preference not to use
> stack, since we got to have kmalloc and error handling in
> i915_sg_create anyway.
> 
> >I've also thought about doing the sg_trim, just never actually tried...
> >
> >Just not convinced that recreating the compact sg is better than copying
> >the already compact sg (for dmabuf, partial).
> 
> Agreed already on dmabuf. Partials yes, I suppose that could be
> implemented more efficiently. You think it is worth special casing
> it rather than reuse the common API? It would have to handle a start
> in the middle of the sg entry so it wouldn't be a straightforward
> copy.

If you happen to running perf and gem_mmap_gtt, you might want to
improve it. (You have to be using lots of large textures with mesa to
worry about it, and then you are more likely bandwidth limited and not
notice the load stalls as much.) It doesn't look too bad:

https://cgit.freedesktop.org/~ickle/linux-2.6/commit/?h=eb-fence&id=6eb7f33c14b9c408856af86c3b35c7ff878afca9

partial pages is a bit special as we have to copy across the dma mapping
as well.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre