linux-kernel - Re: [PATCH v2 01/10] gpu: nova-core: Set correct DMA mask

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Message-ID: <0dbc8f78-5cee-4741-8d33-df3358dd5383@nvidia.com>
Date: Mon, 22 Sep 2025 19:16:53 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>, Alistair Popple <apopple@...dia.com>
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 v2 01/10] gpu: nova-core: Set correct DMA mask

On 9/22/25 9:08 AM, Danilo Krummrich wrote:
> On 9/22/25 1:30 PM, Alistair Popple wrote:
>> +        // SAFETY: No DMA allocations have been made yet
> 
> It's not really about DMA allocations that have been made previously, there is
> no unsafe behavior in that.
> 
> It's about the method must not be called concurrently with any DMA allocation or
> mapping primitives.
> 
> Can you please adjust the comment correspondingly?
> 
>> +        unsafe { pdev.dma_set_mask_and_coherent(DmaMask::new::<47>())? };
> 
> As Boqun mentioned, we shouldn't have a magic number for this. I don't know if
> it will change for future chips, but maybe we should move this to gpu::Spec to

It changes to 52 bits for GH100+ (Hopper/Blackwell+). When I post those
patches, I'll use a HAL to select the value.

> be safe.
> 
> At least, create a constant for it (also in gpu::Spec?); in Nouveau I named this
> NOUVEAU_VA_SPACE_BITS back then. Not a great name, if you have a better idea,
> please go for it. :)

GPU_DMA_BIT_WIDTH, for now?


thanks,
-- 
John Hubbard