linux-kernel - Re: [PATCH v4 08/33] gpu: nova-core: set DMA mask width based on GPU architecture

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Message-Id: <DGC28K8J39E2.1X173NHT9ZJDI@kernel.org>
Date: Wed, 11 Feb 2026 11:28:37 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "John Hubbard" <jhubbard@...dia.com>
Cc: "Alexandre Courbot" <acourbot@...dia.com>, "Joel Fernandes"
 <joelagnelf@...dia.com>, "Timur Tabi" <ttabi@...dia.com>, "Alistair Popple"
 <apopple@...dia.com>, "Eliot Courtney" <ecourtney@...dia.com>, "Zhi Wang"
 <zhiw@...dia.com>, "David Airlie" <airlied@...il.com>, "Simona Vetter"
 <simona@...ll.ch>, "Bjorn Helgaas" <bhelgaas@...gle.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>,
 <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>, "LKML"
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 08/33] gpu: nova-core: set DMA mask width based on
 GPU architecture

On Tue Feb 10, 2026 at 3:45 AM CET, John Hubbard wrote:
> This removes a "TODO" item in the code, which was hardcoded to work on
> Ampere and Ada GPUs. Hopper/Blackwell+ have a larger width, so do an
> early read of boot42, in order to pick the correct value.
>
> Cc: Gary Guo <gary@...yguo.net>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  drivers/gpu/nova-core/driver.rs | 33 ++++++++++++++--------------
>  drivers/gpu/nova-core/gpu.rs    | 38 ++++++++++++++++++++++++---------
>  2 files changed, 44 insertions(+), 27 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
> index e39885c0d5ca..4ff07b643db6 100644
> --- a/drivers/gpu/nova-core/driver.rs
> +++ b/drivers/gpu/nova-core/driver.rs
> @@ -5,7 +5,6 @@
>      device::Core,
>      devres::Devres,
>      dma::Device,
> -    dma::DmaMask,
>      pci,
>      pci::{
>          Class,
> @@ -17,7 +16,10 @@
>      sync::Arc, //
>  };
>  
> -use crate::gpu::Gpu;
> +use crate::gpu::{
> +    Gpu,
> +    Spec, //
> +};
>  
>  #[pin_data]
>  pub(crate) struct NovaCore {
> @@ -29,14 +31,6 @@ pub(crate) struct NovaCore {
>  
>  const BAR0_SIZE: usize = SZ_16M;
>  
> -// For now we only support Ampere which can use up to 47-bit DMA addresses.
> -//
> -// TODO: Add an abstraction for this to support newer GPUs which may support
> -// larger DMA addresses. Limiting these GPUs to smaller address widths won't
> -// have any adverse affects, unless installed on systems which require larger
> -// DMA addresses. These systems should be quite rare.
> -const GPU_DMA_BITS: u32 = 47;
> -
>  pub(crate) type Bar0 = pci::Bar<BAR0_SIZE>;
>  
>  kernel::pci_device_table!(
> @@ -75,18 +69,23 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> impl PinInit<Self, E
>              pdev.enable_device_mem()?;
>              pdev.set_master();
>  
> -            // SAFETY: No concurrent DMA allocations or mappings can be made because
> -            // the device is still being probed and therefore isn't being used by
> -            // other threads of execution.
> -            unsafe { pdev.dma_set_mask_and_coherent(DmaMask::new::<GPU_DMA_BITS>())? };
> -
> -            let bar = Arc::pin_init(

Spurious rename.

> +            let devres_bar = Arc::pin_init(
>                  pdev.iomap_region_sized::<BAR0_SIZE>(0, c"nova-core/bar0"),
>                  GFP_KERNEL,
>              )?;
>  
> +            // Read the GPU spec early to determine the correct DMA address width.

Hm.. we should move the dma_set_mask_and_coherent() call into Gpu::new(), so all
GPU specific initialization remains in the constructor of Gpu.

> +            // Hopper/Blackwell+ support 52-bit DMA addresses, earlier architectures use 47-bit.

I'd move this down to the dma_set_mask_and_coherent() call, or maybe just remove
it as well, since we have the very same comment for Architecture::dma_mask().

> +            let spec = Spec::new(pdev.as_ref(), devres_bar.access(pdev.as_ref())?)?;
> +            dev_info!(pdev.as_ref(), "NVIDIA ({})\n", spec);

This re-introduces pdev.as_ref().

> +
> +            // SAFETY: No concurrent DMA allocations or mappings can be made because
> +            // the device is still being probed and therefore isn't being used by
> +            // other threads of execution.
> +            unsafe { pdev.dma_set_mask_and_coherent(spec.chipset().arch().dma_mask())? };
> +
>              Ok(try_pin_init!(Self {
> -                gpu <- Gpu::new(pdev, bar.clone(), bar.access(pdev.as_ref())?),
> +                gpu <- Gpu::new(pdev, devres_bar.clone(), devres_bar.access(pdev.as_ref())?, spec),
>                  _reg <- auxiliary::Registration::new(
>                      pdev.as_ref(),
>                      c"nova-drm",