[PATCH v4 01/40] drm/gpuvm: Don't require obj lock in destructor path

[Rob Clark]

On Tue, May 20, 2025 at 2:25 PM Dave Airlie <airlied at gmail.com> wrote:
>
> On Sat, 17 May 2025 at 02:20, Rob Clark <robdclark at gmail.com> wrote:
> >
> > On Fri, May 16, 2025 at 2:01 AM Danilo Krummrich <dakr at kernel.org> wrote:
> > >
> > > On Thu, May 15, 2025 at 02:57:46PM -0700, Rob Clark wrote:
> > > > On Thu, May 15, 2025 at 10:55 AM Danilo Krummrich <dakr at kernel.org> wrote:
> > > > > Anyways, I don't agree with that. Even if you can tweak your driver to not run
> > > > > into trouble with this, we can't introduce a mode that violates GOUVM's internal
> > > > > lifetimes and subsequently fix it up with WARN_ON() or BUG_ON().
> > > > >
> > > > > I still don't see a real technical reason why msm can't be reworked to follow
> > > > > those lifetime rules.
> > > >
> > > > The basic issue is that (a) it would be really awkward to have two
> > > > side-by-side VM/VMA management/tracking systems.  But in legacy mode,
> > > > we have the opposite direction of reference holding.  (But at the same
> > > > time, don't need/use most of the features of gpuvm.)
> > >
> > > Ok, let's try to move this forward; I see three options (in order of descending
> > > preference):
> > >
> > >   1) Rework the legacy code to properly work with GPUVM.
> > >   2) Don't use GPUVM for the legacy mode.
> > >   .
> > >   .
> > >   .
> > >   3) Get an ACK from Dave / Sima to implement those workarounds for MSM in
> > >      GPUVM.
> > >
> > > If you go for 3), the code introduced by those two patches should be guarded
> > > with a flag that makes it very clear that this is a workaround specifically
> > > for MSM legacy mode and does not give any guarantees in terms of correctness
> > > regarding lifetimes etc., e.g. DRM_GPUVM_MSM_LEGACY_QUIRK.
> >
> > I'm not even sure how #2 would work, other than just copy/pasta all of
> > drm_gpuvm into msm, which doesn't really seem great.
> >
> > As for #1, even if I could get it to work, it would still be a lot
> > more mmu map/unmap (like on every pageflip, vs the current state that
> > the vma is kept around until the object is freed).  For the
> > non-VM_BIND world, there are advantages to the BO holding the ref to
> > the VMA, rather than the other way around.  Even at just a modest
> > single layer 1080p the map takes ~.2ms and unmap ~.3ms (plus the unmap
> > costs a tlbinv).  So from that standpoint, #3 is the superior option.
> >
>
> Before we get to #3, I'll need a bit more info here on why you have to
> map/unmap the VMA on every pageflip.

Previously we'd keep the VMA hanging around until the GEM obj is
freed.  But that can't work if the VMA (via the VM_BO) is holding a
reference to the GEM obj.

I was kinda thinking about keeping the VMA around until the handle is
closed.. but that doesn't cover the dma-buf case (ie. when you
re-import the dma-buf fd each frame.. I know android does this, unsure
about other wsi's).

> But actually I think 2 is the best option, I think in nouveau this is
> where we ended up, we didn't modify the old submission paths at all
> and kept the old bo/vm lifetimes.
>
> We just added completely new bind/exec ioctls and you can only use one
> method once you've opened an fd.

hmm, but that means tracking VMAs against a single BO differently..
which.. at least seems ugly..

BR,
-R