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

[Joel Fernandes <joelagnelf@nvidia.com>]

On 7/14/2025 10:53 AM, Alice Ryhl wrote:
> On Mon, Jul 14, 2025 at 4:49 PM Joel Fernandes <joelagnelf@nvidia.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@gmail.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.

 - Joel