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

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Message-ID: <378160e3-c081-4a0d-9f58-80a2105de071@nvidia.com>
Date: Tue, 30 Sep 2025 18:53:37 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Alistair Popple <apopple@...dia.com>, 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>,
 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 9/30/25 6:42 PM, Alistair Popple wrote:
> 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

Yes. This will arrive with the Hopper/Blackwell support patchset.

> 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.

Yes. The "soul" of these various patchsets so far is, "get GSP responsive
for Ampere/Ada". To that end, it's OK to temporarily act as if the only
thing that matters in this driver is Ampere/Ada. :)

> 
> 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.

thanks,
-- 
John Hubbard