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

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Message-ID: <9445754d-28d2-4c95-a2f8-b850032b99d1@arm.com>
Date: Thu, 21 Aug 2025 16:02:09 +0100
From: Steven Price <steven.price@....com>
To: Boris Brezillon <boris.brezillon@...labora.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 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.

Thanks,
Steve

[1] Which in this context means well over a decade - it's somewhat scary
how long I've been doing this!

> Regards,
> 
> Boris