[RFC PATCH] drm/panfrost: Add support for mapping BOs on GPU page faults

[Tomeu Vizoso]

On Fri, 14 Jun 2019 at 23:22, Rob Herring <robh at kernel.org> wrote:
>
> On Wed, Jun 12, 2019 at 6:55 AM Tomeu Vizoso <tomeu at tomeuvizoso.net> wrote:
> >
> > On Mon, 10 Jun 2019 at 19:06, Rob Herring <robh at kernel.org> wrote:
> > >
> > > The midgard/bifrost GPUs need to allocate GPU memory which is allocated
> > > on GPU page faults and not pinned in memory. The vendor driver calls
> > > this functionality GROW_ON_GPF.
> > >
> > > This implementation assumes that BOs allocated with the
> > > PANFROST_BO_NOMAP flag are never mmapped or exported. Both of those may
> > > actually work, but I'm unsure if there's some interaction there. It
> > > would cause the whole object to be pinned in memory which would defeat
> > > the point of this.
> > >
> > > Issues/questions/thoughts:
> > >
> > > What's the difference between i_mapping and f_mapping?
> > >
> > > What kind of clean-up on close is needed? Based on vgem faults, there
> > > doesn't seem to be any refcounting. Assume userspace is responsible for
> > > not freeing the BO while a page fault can occur?
> >
> > Aren't we taking a reference on all BOs that a job relates to and
> > unreferencing them once the job is done? I would think that that's
> > enough, or am I missing something?
>
> No, I think we're fine.
>
> > > What about evictions? Need to call mapping_set_unevictable()? Maybe we
> > > want these pages to be swappable, but then we need some notification to
> > > unmap them.
> >
> > I'm not sure there's much point in swapping out pages with lifetimes
> > of a few milliseconds.
>
> The lifetime is *forever* though. If we don't allow swapping, then the
> heap is grow only until the FD is closed. IIRC, the maximum size is on
> the order of 1GB. Seems like you'd want to shrink it with some
> trigger.

I thought that the lifetime of the *contents* of the heap was that of
the job chain that wrote them? Otherwise, only the GPU would know what
can be discarded.

Cheers,

Tomeu