Am 05.04.2017 um 13:13 schrieb Christopher James Halse Rogers:
On Wed, Apr 5, 2017 at 8:14 PM Lucas Stach <l.st...@pengutronix.de
<mailto:l.st...@pengutronix.de>> wrote:
Am Mittwoch, den 05.04.2017, 11:59 +0200 schrieb Daniel Vetter:
> On Wed, Apr 05, 2017 at 10:15:44AM +0200, Lucas Stach wrote:
> > Am Mittwoch, den 05.04.2017, 00:20 +0000 schrieb Christopher
James Halse
> > Rogers:
> > >
> > >
> > > On Tue, Apr 4, 2017 at 9:53 PM Daniel Vetter
<dan...@ffwll.ch <mailto:dan...@ffwll.ch>> wrote:
> > >
> > > On Tue, Apr 4, 2017 at 12:43 PM, Lucas Stach
> > > <l.st...@pengutronix.de
<mailto:l.st...@pengutronix.de>> wrote:
> > > >> If I could guarantee that I'd only ever run on
> > > 4.13-or-later kernels
> > > >> (I think that's when the previous patches will
land?), then
> > > this would
> > > >> indeed be mostly unnecessary. It would save me a
bunch of
> > > addfb calls
> > > >> that would predictably fail, but they're cheap.
> > > >
> > > > I don't think we ever had caps for "things are
working now,
> > > as they are
> > > > supposed to". i915 wasn't properly synchronizing
on foreign
> > > fences for a
> > > > long time, yet we didn't gain a cap for "cross
device sync
> > > works now".
> > > >
> > > > If your distro use-case relies on those things
working it's
> > > probably
> > > > best to just backport the relevant fixes.
> > >
> > > The even better solution for this is to push the
backports
> > > through
> > > upstream -stable kernels. This stuff here is simple
enough
> > > that we can
> > > do it. Same could have been done for the fairly minimal
> > > fencing fixes
> > > for i915 (at least some of them, the ones in the
page-flip).
> > >
> > > Otherwise we'll end up with tons IM_NOT_BUGGY and
> > > IM_SLIGHTLY_LESS_BUGGY and
> > > IM_NOT_BUGGY_EXCEPT_THIS_BOTCHED_BACKPORT
> > > flags where no one at all knows what they mean,
usage between
> > > different drivers and different userspace is entirely
> > > inconsistent and
> > > they just all add to the confusion. They're just
bugs, lets
> > > treat them
> > > like that.
> > >
> > >
> > > It's not *quite* DRM_CAP_PRIME_SCANOUT_NOT_BUGGY - while the
relevant
> > > hardware allegedly supports it, nouveau/radeon/amdgpu don't
do scanout
> > > of GTT, so the lack of this cap indicates that there's no
point in
> > > trying to call addfb2.
> > >
> > >
> > > But calling addfb2 and it failing is not expensive, so this
is rather
> > > niche.
> > >
> > >
> > > In practice I can just restrict attempting to scanout of
imported
> > > buffers to i915, as that's the only driver that it'll work
on. By the
> > > time nouveau/radeon/amdgpu get patches to scanout of GTT the
fixes
> > > should be old enough that I don't need to care about unfixed
kernels.
> > >
> > So given that most discreet hardware won't ever be able to
scanout from
> > GTT (latency and iso requirements will be hard to meet), can't
we just
> > fix the case of the broken prime sharing when migrating to VRAM?
At least some nouveau and AMD devs seem to think that their hardware
is capable of doing it. Shrug.
Wow, wait a second. Recent AMD GPU can scanout from system memory,
that's true.
But you need to met quite a bunch of special allocation requirements to
do this.
When we are talking about sharing between AMD GPUs, (e.g. both exporter
and importer are AMD hardware) than that might work.
But I think it's unrealistic that an imported BO (created by an external
driver stack) will ever meet those requirements.
Christian.
> >
> > I'm thinking about attaching an exclusive fence to the dma-buf
when the
> > migration to VRAM happens, then when the GPU is done with the
buffer we
> > can either write back any changes to GTT, or just drop the
fence in case
> > the GPU didn't modify the buffer.
>
> We could, but someone needs to type the code for it. There's
also the
> problem that you need to migrate back, and then doing all that
behind
> userspaces back might not be the best idea.
Drivers with separate VRAM and GTT are already doing a lot of
migration
behind the userspaces back. The only issue with dma-buf migration to
VRAM is that you probably don't want to migrate the pages, but
duplicate
them in VRAM, doubling memory consumption with possible OOM. But then
you could alloc the memory on addfb where you are able to return
proper
errors.
I would *love* for the driver to copy the pages for me into VRAM for
scanout, rather than me having to spin up an EGL context and run the
trivial blitting shader across an EGLImage.
Are you offering to do it? :)
I'll still need to, for the short term, assume that only i915 can do
this without breaking, though.
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