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

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Message-Id: <DBD38ILN8ANI.K75X0M10PLNL@nvidia.com>
Date: Wed, 16 Jul 2025 10:28:22 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Joel Fernandes" <joelagnelf@...dia.com>, "Alice Ryhl"
 <aliceryhl@...gle.com>
Cc: "Rhys Lloyd" <krakow20@...il.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 Tue Jul 15, 2025 at 12:22 AM JST, Joel Fernandes wrote:
>
>
> On 7/14/2025 10:53 AM, Alice Ryhl wrote:
>> 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.
> Oh, interesting. I am Ok with the [u8; 4] then. Btw, we do have several
> #[repr(C, packed)] in vbios.rs already.

Yeah, in this particular case this is a module-local struct for which
(AFAICT) we don't need to generate references to, so unless there are
other issues I think making it packed and storing the properly-ordered
u32 at construction time is both simpler and safer.