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

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Message-ID: <tmworgafsb7jgxyk3muoj4plhhc57xrlzy7g7cyqahvrj6ame6@5qykqhbtpswf>
Date: Thu, 9 Oct 2025 09:34:36 +1100
From: Alistair Popple <apopple@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
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 2025-10-08 at 21:30 +1100, Danilo Krummrich <dakr@...nel.org> wrote...
> 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.

Yep. I have added the following for v5:

// TODO: Add an abstraction for this to support newer GPUs which may support
// larger DMA addresses. Restricting these to smaller address widths wont have any
// adverse impacts for now.

> >  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:

Indeed I did sorry.
 
> 	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?

Have changed it to this for v5:

        // 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.

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

Ack, that is my general approach. Whether I adaquately explain how they are met
is a different question :)

> 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
>