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

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Message-Id: <DD53EE5HJUZY.2EMREPXQ9P090@kernel.org>
Date: Mon, 29 Sep 2025 09:06:40 +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 Mon Sep 29, 2025 at 2:19 AM CEST, Alistair Popple wrote:
> On 2025-09-26 at 22:00 +1000, Danilo Krummrich <dakr@...nel.org> wrote...
>> 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?
>
> Not at all. The 47 bit limitation comes from what the DMA engines can physically
> address, whilst the MMU converts virtual addresses to physical DMA addresses.

I'm well aware -- what I'm saying is that the number given to
dma_set_mask_and_coherent() does not necessarily only depend on the physical bus
and DMA controller capabilities.

It may also depend on the MMU, since we still need to be able to map DMA memory
in the GPU's virtual address space.

> So the two address spaces are different and can have different widths. Indeed
> most of our current GPUs have a virtual address space of 49 bits whilst only
> supporting 47 bits of DMA address space.

Now, it seems that in this case the DMA engine is the actual limiting factor,
but is this the case for all architectures or may we have cases where the MMU
(or something else) becomes the limiting factor, e.g. in future architectures?