linux-kernel - Re: [PATCH v4 02/40] drm/gpuvm: Allow VAs to hold soft reference to BOs

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Message-ID: <aCWtINcOUWciwx8L@pollux>
Date: Thu, 15 May 2025 11:00:16 +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 02/40] drm/gpuvm: Allow VAs to hold soft reference to
 BOs

On Wed, May 14, 2025 at 10:53:16AM -0700, Rob Clark wrote:
> From: Rob Clark <robdclark@...omium.org>
> 
> Eases migration for drivers where VAs don't hold hard references to
> their associated BO, avoiding reference loops.
> 
> In particular, msm uses soft references to optimistically keep around
> mappings until the BO is distroyed.  Which obviously won't work if the
> VA (the mapping) is holding a reference to the BO.

Ick! This is all complicated enough. Allow drivers to bypass the proper
reference counting for GEM objects in the context of VM_BO structures seems like
an insane footgun.

I don't understand why MSM would need weak references here. Why does msm need
that, but nouveau, Xe, panthor, PowerVR do not?