linux-kernel - Re: [PATCH v2] gpu: nova-core: vbios: change PmuLookupTableEntry to relax alignment

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Message-ID: <CAH5fLgiFOFm+Lh0K_PDZ2eCFa0EAOMk9GRh0Cgh2sm8fay=qnA@mail.gmail.com>
Date: Mon, 14 Jul 2025 16:53:28 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Rhys Lloyd <krakow20@...il.com>, acourbot@...dia.com, dakr@...nel.org, 
	rust-for-linux@...r.kernel.org, airlied@...il.com, simona@...ll.ch, 
	nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] gpu: nova-core: vbios: change PmuLookupTableEntry to
 relax alignment

On Mon, Jul 14, 2025 at 4:49 PM Joel Fernandes <joelagnelf@...dia.com> wrote:
>
> Hello, Rhys,
>
> On Mon, Jul 14, 2025 at 04:02:23AM -0700, Rhys Lloyd wrote:
> > Instead of the data field containing a u32 and changing the alignment,
> > change data to [u8; 4] and convert to u32 with a helper function.
> > Removes another magic number by making the struct the same size as
> > the data it needs to read, allowing the use of
> > `size_of::<PmuLookupTableEntry>()`
> >
> > Signed-off-by: Rhys Lloyd <krakow20@...il.com>
> > ---
> > Changes in v2:
> > - get_data helper function renamed to data
> >
> > ---
> >  drivers/gpu/nova-core/vbios.rs | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/nova-core/vbios.rs b/drivers/gpu/nova-core/vbios.rs
> > index 5b5d9f38cbb3..339c66e63c7e 100644
> > --- a/drivers/gpu/nova-core/vbios.rs
> > +++ b/drivers/gpu/nova-core/vbios.rs
> > @@ -896,21 +896,26 @@ fn try_from(base: BiosImageBase) -> Result<Self> {
> >  struct PmuLookupTableEntry {
> >      application_id: u8,
> >      target_id: u8,
> > -    data: u32,
> > +    data: [u8; 4],
>
> Instead of this, could we make the struct as #repr[(C, packed)] or does that
> not work for some reason?

It would probably, but packed structs aren't very nice to work with
because Rust has to be really careful to never generate a reference to
unaligned fields.

Alice