linux-kernel - Re: [PATCH v3 01/13] gpu: nova-core: Set correct DMA mask

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Message-ID: <ljesevy2hkryskvrtjtjzubybzc2o35wdrz6tgsodc6ljiv27j@mvyi6gqiryvz>
Date: Wed, 1 Oct 2025 11:42:39 +1000
From: Alistair Popple <apopple@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: rust-for-linux@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	acourbot@...dia.com, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
	John Hubbard <jhubbard@...dia.com>, Joel Fernandes <joelagnelf@...dia.com>, 
	Timur Tabi <ttabi@...dia.com>, linux-kernel@...r.kernel.org, nouveau@...ts.freedesktop.org
Subject: Re: [PATCH v3 01/13] gpu: nova-core: Set correct DMA mask

On 2025-09-30 at 23:29 +1000, Danilo Krummrich <dakr@...nel.org> wrote...
> On 9/30/25 3:16 PM, Alistair Popple wrote:
> > +        // SAFETY: No DMA allocations have been made yet
> > +        unsafe { pdev.dma_set_mask_and_coherent(DmaMask::new::<47>())? };
> 
> I think you forgot to derive the value from the relevant sources, i.e. physical
> bus, DMA controller and MMU capabilities.
> 
> I assume not all GPU architectures / generations have the exact same capabilities?

Right. Long term we need a HAL for this, and I believe John was going to look
at that. In the short term everything we care about supports the same 47-bit
address width. And when I say "everything" I mean Turing and Ampere, although
I'm pretty sure 47-bits goes back to at least Pascal.

Newer GPU architectures (Hopper+) support greater widths (52-bits), but there's
no real impact to constraining these for now until a proper HAL is in place.

And that sounds like an excellent addition that I should make to the commit
logs and/or the comment on the constant definition, which fell off my radar when
reworking the rest of this series but will fix for v4.