linux-kernel - Re: [PATCH drm-misc-next v3 6/7] drm/gpuvm: generalize dma

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Message-ID: <20230913091918.62c06a30@collabora.com>
Date:   Wed, 13 Sep 2023 09:19:18 +0200
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Dave Airlie <airlied@...il.com>
Cc:     Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>,
        Danilo Krummrich <dakr@...hat.com>, daniel@...ll.ch,
        matthew.brost@...el.com, sarah.walker@...tec.com,
        donald.robson@...tec.com, christian.koenig@....com,
        faith.ekstrand@...labora.com, dri-devel@...ts.freedesktop.org,
        nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v3 6/7] drm/gpuvm: generalize
 dma_resv/extobj handling and GEM validation

On Wed, 13 Sep 2023 17:05:42 +1000
Dave Airlie <airlied@...il.com> wrote:

> On Wed, 13 Sept 2023 at 17:03, Boris Brezillon
> <boris.brezillon@...labora.com> wrote:
> >
> > On Tue, 12 Sep 2023 18:20:32 +0200
> > Thomas Hellström <thomas.hellstrom@...ux.intel.com> wrote:
> >  
> > > > +/**
> > > > + * get_next_vm_bo_from_list() - get the next vm_bo element
> > > > + * @__gpuvm: The GPU VM
> > > > + * @__list_name: The name of the list we're iterating on
> > > > + * @__local_list: A pointer to the local list used to store already iterated items
> > > > + * @__prev_vm_bo: The previous element we got from drm_gpuvm_get_next_cached_vm_bo()
> > > > + *
> > > > + * This helper is here to provide lockless list iteration. Lockless as in, the
> > > > + * iterator releases the lock immediately after picking the first element from
> > > > + * the list, so list insertion deletion can happen concurrently.  
> > >
> > > Are the list spinlocks needed for that async state update from within
> > > the dma-fence critical section we've discussed previously?  
> >
> > Any driver calling _[un]link() from its drm_gpu_scheduler::run_job()
> > hook will be in this situation (Panthor at the moment, PowerVR soon). I
> > get that Xe and Nouveau don't need that because they update the VM
> > state early (in the ioctl path), but I keep thinking this will hurt us
> > if we don't think it through from the beginning, because once you've
> > set this logic to depend only on resv locks, it will be pretty hard to
> > get back to a solution which lets synchronous VM_BINDs take precedence
> > on asynchronous request, and, with vkQueueBindSparse() passing external
> > deps (plus the fact the VM_BIND queue might be pretty deep), it can
> > take a long time to get your synchronous VM_BIND executed...  
> 
> btw what is the use case for this? do we have actual vulkan
> applications we know will have problems here?

I don't, but I think that's a concern Faith raised at some point (dates
back from when I was reading threads describing how VM_BIND on i915
should work, and I was clearly discovering this whole VM_BIND thing at
that time, so maybe I misunderstood).

> 
> it feels like a bit of premature optimisation, but maybe we have use cases.

Might be, but that's the sort of thing that would put us in a corner if
we don't have a plan for when the needs arise. Besides, if we don't
want to support that case because it's too complicated, I'd recommend
dropping all the drm_gpuvm APIs that let people think this mode is
valid/supported (map/remap/unmap hooks in drm_gpuvm_ops,
drm_gpuvm_sm_[un]map helpers, etc). Keeping them around just adds to the
confusion.