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

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Message-ID: <aCb-72KH-NrzvGXy@pollux>
Date: Fri, 16 May 2025 11:01:35 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Rob Clark <robdclark@...il.com>
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 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.

- Danilo