linux-kernel - Re: [PATCH v4 01/13] gpu: nova-core: Set correct DMA mask

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Message-Id: <DDCVDASJNW9T.BUT6XK1WXD0A@kernel.org>
Date: Wed, 08 Oct 2025 12:30:25 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "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>, "John Hubbard"
 <jhubbard@...dia.com>, "Joel Fernandes" <joelagnelf@...dia.com>, "Timur
 Tabi" <ttabi@...dia.com>, <linux-kernel@...r.kernel.org>,
 <nouveau@...ts.freedesktop.org>
Subject: Re: [PATCH v4 01/13] gpu: nova-core: Set correct DMA mask

On Wed Oct 8, 2025 at 2:12 AM CEST, Alistair Popple wrote:
> Set the correct DMA mask. Without this DMA will fail on some setups.
>
> Signed-off-by: Alistair Popple <apopple@...dia.com>
>
> ---
>
> Changes for v4:
>  - Use a const (GPU_DMA_BITS) instead of a magic number
>
> Changes for v2:
>  - Update DMA mask to correct value for Ampere/Turing (47 bits)
> ---
>  drivers/gpu/nova-core/driver.rs | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
> index edc72052e27a..84fe4a45eb6a 100644
> --- a/drivers/gpu/nova-core/driver.rs
> +++ b/drivers/gpu/nova-core/driver.rs
> @@ -3,6 +3,8 @@
>  use kernel::{
>      auxiliary, c_str,
>      device::Core,
> +    dma::Device,
> +    dma::DmaMask,
>      pci,
>      pci::{Class, ClassMask, Vendor},
>      prelude::*,
> @@ -20,6 +22,10 @@ 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.
> +const GPU_DMA_BITS: u32 = 47;

IIRC, the idea was to abstract this properly with a subsequent patch worked on
by John. In that case, please add a TODO.

>  pub(crate) type Bar0 = pci::Bar<BAR0_SIZE>;
>  
>  kernel::pci_device_table!(
> @@ -57,6 +63,9 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self
>          pdev.enable_device_mem()?;
>          pdev.set_master();
>  
> +        // SAFETY: No DMA allocations have been made yet

I think you forgot to address my comment from v2:

	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?

In general, I recommend having a look at the safety requirements of the
corresponding function.

NIT: Please end with a period.

> +        unsafe { pdev.dma_set_mask_and_coherent(DmaMask::new::<GPU_DMA_BITS>())? };
> +
>          let devres_bar = Arc::pin_init(
>              pdev.iomap_region_sized::<BAR0_SIZE>(0, c_str!("nova-core/bar0")),
>              GFP_KERNEL,
> -- 
> 2.50.1