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Date: Thu, 05 Feb 2026 15:19:49 +0100
Message-Id: <DG73EB8091J4.3SFPDC5TLH0M7@kernel.org>
Subject: Re: [PATCH] gpu: nova-core: fix aux device registration for
 multi-GPU systems
Cc: "Gary Guo" <gary@garyguo.net>, "Alexandre Courbot"
 <acourbot@nvidia.com>, "Joel Fernandes" <joelagnelf@nvidia.com>, "Timur
 Tabi" <ttabi@nvidia.com>, "Alistair Popple" <apopple@nvidia.com>, "Eliot
 Courtney" <ecourtney@nvidia.com>, "Zhi Wang" <zhiw@nvidia.com>, "David
 Airlie" <airlied@gmail.com>, "Simona Vetter" <simona@ffwll.ch>, "Bjorn
 Helgaas" <bhelgaas@google.com>, "Miguel Ojeda" <ojeda@kernel.org>, "Alex
 Gaynor" <alex.gaynor@gmail.com>, "Boqun Feng" <boqun.feng@gmail.com>,
 =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, "Benno Lossin"
 <lossin@kernel.org>, "Andreas Hindborg" <a.hindborg@kernel.org>, "Alice
 Ryhl" <aliceryhl@google.com>, "Trevor Gross" <tmgross@umich.edu>,
 <nouveau@lists.freedesktop.org>, <rust-for-linux@vger.kernel.org>, "LKML"
 <linux-kernel@vger.kernel.org>
To: "John Hubbard" <jhubbard@nvidia.com>, "Matthew Wilcox"
 <willy@infradead.org>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20260205041120.127106-1-jhubbard@nvidia.com>
 <DG72N8GDQ9E0.2FUX560O3I19X@garyguo.net>
 <aYSfpf3xZNwZhzXs@casper.infradead.org>
In-Reply-To: <aYSfpf3xZNwZhzXs@casper.infradead.org>

On Thu Feb 5, 2026 at 2:48 PM CET, Matthew Wilcox wrote:
> On Thu, Feb 05, 2026 at 01:44:27PM +0000, Gary Guo wrote:
>> > Fix this by using an atomic counter to generate unique IDs for each
>> > GPU's aux device registration. The TODO item to eventually use XArray
>> > for recycling aux device IDs is retained, but for now, this works very
>> > nicely.
>> >
>> > This has the side effect of making debugfs[1] work on multi-GPU system=
s.
>>=20
>> Hi John,
>>=20
>> Looks like this is something that should be achieved via IDA?
>
> Yes, if you have no need to go from ID to pointer, an IDA is better.
> That said, as far as I understand what this code is doing, an atomic_t
> solves the problem just fine and is cheaper.

I agree, for now an atomic should be perfectly fine. Though, with enough
patience binding/unbinding the driver from sysfs you can probably make this
overflow. :)

The reason for the Xarray TODO is that it is one option for a place where
nova-core can store nova-drm / vGPU specific data, once either vGPU or nova=
-drm
attaches to the auxiliary device. But I think there may be better alternati=
ves.