linux-kernel - Re: [PATCH v4 1/7] drm/panthor: Add support for atomic page table updates

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Message-ID: <20250821171519.7224f470@fedora>
Date: Thu, 21 Aug 2025 17:15:19 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Steven Price <steven.price@....com>
Cc: Caterina Shablia <caterina.shablia@...labora.com>, 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>, Frank Binns <frank.binns@...tec.com>, Matt
 Coster <matt.coster@...tec.com>, Karol Herbst <kherbst@...hat.com>, Lyude
 Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>, Liviu Dudau
 <liviu.dudau@....com>, Lucas De Marchi <lucas.demarchi@...el.com>, Thomas
 Hellström <thomas.hellstrom@...ux.intel.com>, Rodrigo
 Vivi <rodrigo.vivi@...el.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, nouveau@...ts.freedesktop.org,
 intel-xe@...ts.freedesktop.org, asahi@...ts.linux.dev, Asahi Lina
 <lina@...hilina.net>
Subject: Re: [PATCH v4 1/7] drm/panthor: Add support for atomic page table
 updates

On Thu, 21 Aug 2025 16:02:09 +0100
Steven Price <steven.price@....com> wrote:

> On 21/08/2025 12:51, Boris Brezillon wrote:
> > On Wed, 16 Jul 2025 16:43:24 +0100
> > Steven Price <steven.price@....com> wrote:  
> [...]
> >> Although in general I'm a bit wary of relying on the whole lock region
> >> feature - previous GPUs have an errata. But maybe I'm being over
> >> cautious there.  
> > 
> > We're heavily relying on it already to allow updates of the VM while
> > the GPU is executing stuff. If that's problematic on v10+, I'd rather
> > know early :D.  
> 
> I think I'm just scarred by my experiences over a decade ago... ;)
> 
> I'm not aware of any issues with the modern[1] GPUs. The issue used to
> be that the lock region could get accidentally unlocked by a cache flush
> from another source - specifically the cache flush on job start flag.
> 
> It's also not a major issue if you keep the page tables consistent, the
> lock region in theory allows a region to be in an inconsistent state -
> but generally there's no need for that. AFAIK we mostly keep the tables
> consistent anyway.

Right, it's not a problem until we introduce sparse binding support, at
which point atomicity becomes important, and given remapping is not a
thing the io-pagetable layer provides (remap has to be unmap+map), I
need to rely on region locking to make it work, or we'll have to eat the
fault-but-not-really-because-its-being-remapped overhead/complexity.
Honestly, I'd rather rely on region locking if it's working, because
it's far simpler ;-).