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

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Message-Id: <DD2PRD2XEZRE.1YACAPZWRYLZO@kernel.org>
Date: Fri, 26 Sep 2025 14:00:01 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alistair Popple" <apopple@...dia.com>
Cc: "John Hubbard" <jhubbard@...dia.com>, <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 Tue Sep 23, 2025 at 6:29 AM CEST, Alistair Popple wrote:
> On 2025-09-23 at 12:16 +1000, John Hubbard <jhubbard@...dia.com> wrote...
>> 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?
>
> Sure.
>
>> >> +        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. :)
>
> Well it's certainly not the VA_SPACE width ... that's a different address space :-)

I mean, sure. But isn't the limitation of 47 bits coming from the MMU and hence
defines the VA space width and the DMA bit width we can support?

> I thought from the context that the magic number was pretty obviously the
> supported DMA address width in bits, especially given the extra decoration
> of the DmaMask type. Certainly that's been the accepted practice for the rest
> of the kernel where pretty much all drivers just use something of the form
> dma_set_mask(drm_dev->dev, DMA_BIT_MASK(44)) or whatever DMA address widths
> they support.
>
>> GPU_DMA_BIT_WIDTH, for now?
>
> Works for me.
>
>> thanks,
>> -- 
>> John Hubbard
>>