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

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Message-ID: <CAF6AEGu=KzCnkxuUsYvCHBGwo-e2W16u_cRT1NFAXLphty1_ig@mail.gmail.com>
Date: Fri, 16 May 2025 09:20:27 -0700
From: Rob Clark <robdclark@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org, 
	linux-arm-msm@...r.kernel.org, Connor Abbott <cwabbott0@...il.com>, 
	Rob Clark <robdclark@...omium.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 01/40] drm/gpuvm: Don't require obj lock in destructor path

On Fri, May 16, 2025 at 2:01 AM Danilo Krummrich <dakr@...nel.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@...nel.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.

BR,
-R