[PATCH v3 0/8] drm: Introduce sparse GEM shmem

[Boris Brezillon]

Hi Christian,

On Thu, 10 Apr 2025 17:05:07 +0200
Christian König <christian.koenig at amd.com> wrote:

> Hi Boris,
> 
> thanks for looping me in. Can you also send the full patch set to me since I don't see that on the mailing list (yet maybe).
> 
> Am 10.04.25 um 16:48 schrieb Boris Brezillon:
> > +Christian, Alyssa and Faith, as suggested by Sima. I'll make sure to
> > Cc you on v4, but before that, I'd like to get your opinion on the
> > drm-gem/drm-gem-shmem changes to see if sending a v4 is actually
> > desirable or if I should go back to the drawing board.
> >
> > On Fri,  4 Apr 2025 11:26:26 +0200
> > Boris Brezillon <boris.brezillon at collabora.com> wrote:
> >  
> >> This patch series is a proposal for implementing sparse page allocations
> >> for shmem objects. It was initially motivated by a kind of BO managed by
> >> the Panfrost/Panthor and Lima drivers, the tiler heap, which grows on
> >> demand every time the GPU faults on a virtual address within the heap BO
> >> range.  
> 
> Oh, wait a second! GPU faults and DMA fences are usually fundamentally incompatible.
> 
> So stuff like filling in GEM objects on demand like you suggest here is usually seen as illegal. All resources must be pre-pinned before the submission.

Unfortunately, that's already how it's done in lima, panfrost and
panthor.

> 
> Faulting is only legal when you have something like HMM, SVM or whatever you call it. And then you can just use a plain shmem object to provide you with backing pages.
> 
> I mean we could in theory allow faulting on GEM objects as well, but we would need to take very strict precautions on that we currently don't have as far as I know.

We only use this mechanism for very specific allocations: tiler memory
whose maximum size can't be guessed upfront because tile binning is by
nature unpredictible.

> 
> So could you explain how this works in the first place?

I can explain you how this works in Panthor, yes. You get an initial
amount of memory that the tiler can use, when it runs out of memory, it
will first ask the system for more memory, if the allocation fails, it
will fallback to what they call "incremental rendering", where the
already binned primitives are flushed to the FB in order to free memory,
and the rendering starts over from there, with the memory that has been
freed.

In Panthor, this on-demand allocation scheme is something that allows
us to speed-up rendering when there's memory available, but we can make
progress when that's not the case, hence the failable allocation scheme
I'm proposing here.

In Panfrost and Lima, we don't have this concept of "incremental
rendering", so when we fail the allocation, we just fail the GPU job
with an unhandled GPU fault. And that's how it is today, the
alloc-on-fault mechanism is being used in at least 3 drivers, and all
I'm trying to do here is standardize it and try to document the
constraints that comes with this model, constraint that are currently
being ignored. Like the fact allocations in the fault handler path
shouldn't block so we're guaranteed to signal the job fence in finite
time, and we don't risk a deadlock between the driver shrinker and the
job triggering the fault.

I'm well aware of the implications of what I'm proposing here, but
ignoring the fact some drivers already violate the rules don't make them
disappear. So I'd rather work with you and others to clearly state what
the alloc-in-fault-path rules are, and enforce them in some helper
functions, than pretend these ugly things don't exist. :D

Regards,

Boris