[PATCH] drm: modify pages_to_sg prime helper to create optimized SG table

[Rahul Sharma]

On Thu, Jan 31, 2013 at 5:47 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Thu, Jan 31, 2013 at 12:54 PM, Rahul Sharma <r.sh.open at gmail.com> wrote:
>> I have parsed the related code and it looks fine to me. I couldn't find
>> any code section, expecting sg-tables with single-page sgl entries. I
>> just want to ensure again that it doesn't cause any side effects on
>> various platforms.
>
> Just chatted with Chris Wilson on our team, and at last i915.ko _has_
> code paths which rely on this. See the reloc handling offset
> computations in i915_gem_execbuf.c. I haven't checked
> ttm/radeon/nouveau/vmwgfx simply because I don't have the time right
> now, and I know that doing this carefully will blow through a few
> days. So this patch will break prime buffer sharing on the desktop.
>
> The other thing is that only doing sg entry coalescing for
> prime/dma_buf will lead to a QA problem, since all the normal sg
> tables still have a 1:1 relationship. So imo the right way to move
> forward with this is to convert the various sg table constructors in
> i915/ttm/radeon/nouveau/... over to coalesce pages. This allows us to
> fix any fallout step-by-step.
>
> Then, once each driver is ready for this, we can merge your patch.
>

Looks fine to me.

>>> For the patch itself I think there's now a generic pages_to_sg helper
>>> in the dma core which does exactly what you want it to do. I can dig
>>> it out if you can't find it.
>>>
>>
>> Sorry, I din't get this part. Please elaborate a bit.
>
> See sg_alloc_table_from_pages in lib/scatterlist.c introduced with:
>
> commit efc42bc98058a36d761b16a114823db1a902ed05
> Author: Tomasz Stanislawski <t.stanislaws at samsung.com>
> Date:   Mon Jun 18 09:25:01 2012 +0200
>
>     scatterlist: add sg_alloc_table_from_pages function
>

Yes, my patch is using sg_alloc_table_from_pages.

regards,
Rahul Sharma.

> Cheers, Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch