[PATCH v3 0/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[PATCH v3 0/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[Alexandre Courbot]

This is useful in Nova's GSP message handling, as some messages are
empty and we currently need to explicitly use an empty structure for
them.

If accepted, I would like to merge it through `drm-rust-next` so Nova
code can start using this feature quickly.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
Changes in v3:
- Use better safety statement. (thanks Gary!)
- Also add `PhantomData`. (thanks Gary!)
- Link to v2: https://patch.msgid.link/20251208-transmute_unit-v2-0-aa17a6848afb@nvidia.com

Changes in v2:
- Make use of new feature in Nova.
- Link to v1: https://patch.msgid.link/20251208-transmute_unit-v1-1-680c7386b5d9@nvidia.com

---
Alexandre Courbot (2):
      rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs
      gpu: nova-core: gsp: use () as message type for GspInitDone message

 drivers/gpu/nova-core/gsp/commands.rs | 6 +++---
 rust/kernel/transmute.rs              | 8 ++++++++
 2 files changed, 11 insertions(+), 3 deletions(-)
---
base-commit: ba65a4e7120a616d9c592750d9147f6dcafedffa
change-id: 20251208-transmute_unit-78ab58ba9e6e

Best regards,
-- 
Alexandre Courbot <acourbot@nvidia.com>

[PATCH v3 1/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[Alexandre Courbot]

This is useful when using types that may or may not be empty in generic
code relying on these traits. It is also safe because technically a
no-op.

Reviewed-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/transmute.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index be5dbf3829e2..a888a312e7ff 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -170,6 +170,10 @@ macro_rules! impl_frombytes {
 }
 
 impl_frombytes! {
+    // SAFETY: Inhabited ZSTs only have one possible bit pattern.
+    (),
+    {<T>} core::marker::PhantomData<T>,
+
     // SAFETY: All bit patterns are acceptable values of the types below.
     u8, u16, u32, u64, usize,
     i8, i16, i32, i64, isize,
@@ -230,6 +234,10 @@ macro_rules! impl_asbytes {
 }
 
 impl_asbytes! {
+    // SAFETY: Inhabited ZSTs only have one possible bit pattern.
+    (),
+    {<T>} core::marker::PhantomData<T>,
+
     // SAFETY: Instances of the following types have no uninitialized portions.
     u8, u16, u32, u64, usize,
     i8, i16, i32, i64, isize,

-- 
2.52.0

[PATCH v3 2/2] gpu: nova-core: gsp: use () as message type for GspInitDone message

[Alexandre Courbot]

`GspInitDone` has no payload whatsoever, so the unit type `()` is the
correct way to represent its message content. We can use it now that
`()` implements `FromBytes`.

Reviewed-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 drivers/gpu/nova-core/gsp/commands.rs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/nova-core/gsp/commands.rs b/drivers/gpu/nova-core/gsp/commands.rs
index 0425c65b5d6f..2050771f9b53 100644
--- a/drivers/gpu/nova-core/gsp/commands.rs
+++ b/drivers/gpu/nova-core/gsp/commands.rs
@@ -142,7 +142,7 @@ fn init_variable_payload(
 }
 
 /// Message type for GSP initialization done notification.
-struct GspInitDone {}
+struct GspInitDone;
 
 // SAFETY: `GspInitDone` is a zero-sized type with no bytes, therefore it
 // trivially has no uninitialized bytes.
@@ -151,13 +151,13 @@ unsafe impl FromBytes for GspInitDone {}
 impl MessageFromGsp for GspInitDone {
     const FUNCTION: MsgFunction = MsgFunction::GspInitDone;
     type InitError = Infallible;
-    type Message = GspInitDone;
+    type Message = ();
 
     fn read(
         _msg: &Self::Message,
         _sbuffer: &mut SBufferIter<array::IntoIter<&[u8], 2>>,
     ) -> Result<Self, Self::InitError> {
-        Ok(GspInitDone {})
+        Ok(GspInitDone)
     }
 }
 

-- 
2.52.0

Re: [PATCH v3 1/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[Benno Lossin]

On Tue Dec 9, 2025 at 3:57 AM CET, Alexandre Courbot wrote:
> This is useful when using types that may or may not be empty in generic
> code relying on these traits. It is also safe because technically a
> no-op.
>
> Reviewed-by: Alistair Popple <apopple@nvidia.com>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
>  rust/kernel/transmute.rs | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index be5dbf3829e2..a888a312e7ff 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -170,6 +170,10 @@ macro_rules! impl_frombytes {
>  }
>  
>  impl_frombytes! {
> +    // SAFETY: Inhabited ZSTs only have one possible bit pattern.

Even inhabited ZSTs cannot just be conjured out of thin air, since there
might be safety invariants on the creation of such a type. Now these
two concrete ones do not have any, so this is fine, but should still be
mentioned in the comment IMO.

Cheers,
Benno

> +    (),
> +    {<T>} core::marker::PhantomData<T>,
> +
>      // SAFETY: All bit patterns are acceptable values of the types below.
>      u8, u16, u32, u64, usize,
>      i8, i16, i32, i64, isize,

Re: [PATCH v3 0/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[Gary Guo]

On Tue, 09 Dec 2025 11:57:58 +0900
Alexandre Courbot <acourbot@nvidia.com> wrote:

> This is useful in Nova's GSP message handling, as some messages are
> empty and we currently need to explicitly use an empty structure for
> them.
> 
> If accepted, I would like to merge it through `drm-rust-next` so Nova
> code can start using this feature quickly.
> 
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
> Changes in v3:
> - Use better safety statement. (thanks Gary!)
> - Also add `PhantomData`. (thanks Gary!)
> - Link to v2: https://patch.msgid.link/20251208-transmute_unit-v2-0-aa17a6848afb@nvidia.com
> 
> Changes in v2:
> - Make use of new feature in Nova.
> - Link to v1: https://patch.msgid.link/20251208-transmute_unit-v1-1-680c7386b5d9@nvidia.com
> 
> ---
> Alexandre Courbot (2):
>       rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs
>       gpu: nova-core: gsp: use () as message type for GspInitDone message
> 
>  drivers/gpu/nova-core/gsp/commands.rs | 6 +++---
>  rust/kernel/transmute.rs              | 8 ++++++++
>  2 files changed, 11 insertions(+), 3 deletions(-)
> ---
> base-commit: ba65a4e7120a616d9c592750d9147f6dcafedffa
> change-id: 20251208-transmute_unit-78ab58ba9e6e
> 
> Best regards,

Re: [PATCH v3 1/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[Alice Ryhl]

On Tue, Dec 09, 2025 at 01:14:08PM +0100, Benno Lossin wrote:
> On Tue Dec 9, 2025 at 3:57 AM CET, Alexandre Courbot wrote:
> > This is useful when using types that may or may not be empty in generic
> > code relying on these traits. It is also safe because technically a
> > no-op.
> >
> > Reviewed-by: Alistair Popple <apopple@nvidia.com>
> > Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> > ---
> >  rust/kernel/transmute.rs | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> > index be5dbf3829e2..a888a312e7ff 100644
> > --- a/rust/kernel/transmute.rs
> > +++ b/rust/kernel/transmute.rs
> > @@ -170,6 +170,10 @@ macro_rules! impl_frombytes {
> >  }
> >  
> >  impl_frombytes! {
> > +    // SAFETY: Inhabited ZSTs only have one possible bit pattern.
> 
> Even inhabited ZSTs cannot just be conjured out of thin air, since there
> might be safety invariants on the creation of such a type. Now these
> two concrete ones do not have any, so this is fine, but should still be
> mentioned in the comment IMO.

I think we can say "it has no invariants".

Alice