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

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Message-ID: <b5f42338-e5b3-4823-8aa9-9374c22f1209@nvidia.com>
Date: Fri, 29 Aug 2025 16:55:40 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Alistair Popple <apopple@...dia.com>, dri-devel@...ts.freedesktop.org,
 dakr@...nel.org, acourbot@...dia.com
Cc: 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 01/10] gpu: nova-core: Set correct DMA mask

On 8/27/25 1:19 AM, Alistair Popple wrote:
> Set the correct DMA mask. Without this DMA will fail on some setups.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> ---
>  drivers/gpu/nova-core/driver.rs | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
> index 274989ea1fb4a..3e154ffb6be4b 100644
> --- a/drivers/gpu/nova-core/driver.rs
> +++ b/drivers/gpu/nova-core/driver.rs
> @@ -1,6 +1,9 @@
>  // SPDX-License-Identifier: GPL-2.0
>  
> -use kernel::{auxiliary, bindings, c_str, device::Core, pci, prelude::*, sizes::SZ_16M, sync::Arc};
> +use kernel::{
> +    auxiliary, bindings, c_str, device::Core, dma::Device, dma::DmaMask, pci, prelude::*,
> +    sizes::SZ_16M, sync::Arc,
> +};
>  
>  use crate::gpu::Gpu;
>  
> @@ -34,6 +37,9 @@ 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
> +        unsafe { pdev.dma_set_mask_and_coherent(DmaMask::new::<48>())? };

Eventually, should be 52 bits wide, rather than 48. Or so I believe from
looking at various drivers, including Nouveau (which uses 52-bit for 
Blackwell) and mlx* (which use a 64-bit mask).

However, it works for the current series, because this series only supports
Ampere GPUs, and 48 bits suffices for those.

So, you could leave this patch as-is, and I'll change 48 --> 52 in the
upcoming Hopper/Blackwell series. Or you can change it now.

Either way is fine, so:


Reviewed-by: John Hubbard <jhubbard@...dia.com>

thanks,
-- 
John Hubbard

> +
>          let bar = Arc::pin_init(
>              pdev.iomap_region_sized::<BAR0_SIZE>(0, c_str!("nova-core/bar0")),
>              GFP_KERNEL,