[PATCH 1/2] drm/amdgpu: Enable scatter gather display support

[Li, Samuel]

How about that, in my previous patches, it actually allows three scenarios,
1) VRAM as display buffer
2) GTT as display buffer
3) Mixed GTT/display buffer, as you have asked.

I think the first step is to make the 2nd scenario work at optimal level. After that, if anyone wants to work the 3rd scenario, I have no objection at all.

Regards,
Samuel Li


> -----Original Message-----
> From: Samuel Li [mailto:samuel.li at amd.com]
> Sent: Wednesday, March 07, 2018 1:54 PM
> To: Alex Deucher <alexdeucher at gmail.com>
> Cc: Michel Dänzer <michel at daenzer.net>; Koenig, Christian
> <Christian.Koenig at amd.com>; amd-gfx list <amd-gfx at lists.freedesktop.org>
> Subject: Re: [PATCH 1/2] drm/amdgpu: Enable scatter gather display support
> 
> > You might also want to prefer VRAM, but also allow buffers to fall
> > back to GTT if necessary.
> For display buffer, this case seems not really attractive. When display buffer
> changes between GTT and VRAM dynamically, our driver needs to adpat too,
> which is hard to see the benefits and not really worth the effort.
> 
> Sam
> 
> 
> 
> On 2018-03-07 01:27 PM, Alex Deucher wrote:
> > On Wed, Mar 7, 2018 at 1:18 PM, Samuel Li <samuel.li at amd.com> wrote:
> >> I think it's not useful though. Think about that, SG display feature is
> intended to use as less VRAM as possible. Will someone want a display
> buffer sometimes VRAM, sometimes GTT?
> >> Hardly a case to me, and I think it's a waste of effort. That also might
> explain no driver does that now.
> >
> > You might want different strategies depending on how much VRAM you
> > have.  If you have a bunch and it performs better, prefer it.  If you
> > have limited VRAM, you might want to prefer GTT.  You might also want
> > to prefer VRAM, but also allow buffers to fall back to GTT if
> > necessary.  This would make the logic dynamic and all in one place.
> > The kernel can advertise what asics support scanout from system ram
> > and then UMDs can just query that to choose placements rather than
> > adding hardcoded logic for specific asics.
> >
> > Alex
> >
> >>
> >> Sam
> >>
> >>
> >>
> >> On 2018-03-07 12:50 PM, Michel Dänzer wrote:
> >>> On 2018-03-07 06:38 PM, Alex Deucher wrote:
> >>>> On Wed, Mar 7, 2018 at 12:29 PM, Samuel Li <samuel.li at amd.com>
> wrote:
> >>>>>
> >>>>> Why so complicated? If old user space compatibility is required, just
> use sg_display=0.
> >>>>
> >>>> It will always just work in that case and we can adjust for the
> >>>> optimal scenario by default in all cases without requiring the user
> >>>> to set misc parameters to tune their setup that may break in the
> >>>> future or hide bugs because a user sets it and forgets it.
> >>>
> >>> Right. I don't agree it's all that complicated anyway, we do this
> >>> sort of thing all the time, it also makes our lives easier down the road.
> >>>
> >>>