[PATCH v4 0/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[PATCH v4 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 v4:
- Improve safety comment to mention lack of invariants.
- Link to v3: https://patch.msgid.link/20251209-transmute_unit-v3-0-819fe584ba06@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: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
change-id: 20251208-transmute_unit-78ab58ba9e6e

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

[PATCH v4 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>
Reviewed-by: Gary Guo <gary@garyguo.net>
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..5711580c9f9b 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, and these two have no invariant.
+    (),
+    {<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, and these two have no invariant.
+    (),
+    {<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 v4 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 v4 1/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[Benno Lossin]

On Mon Dec 15, 2025 at 6:49 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>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

Reviewed-by: Benno Lossin <lossin@kernel.org>

Cheers,
Benno

> ---
>  rust/kernel/transmute.rs | 8 ++++++++
>  1 file changed, 8 insertions(+)

Re: [PATCH v4 1/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[Gary Guo]

On Mon, 15 Dec 2025 14:49:09 +0900
Alexandre Courbot <acourbot@nvidia.com> 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>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> 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..5711580c9f9b 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, and these two have no invariant.
> +    (),
> +    {<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, and these two have no invariant.

Some nit: technically you can turn any ZST into bytes, even those with
invariants or uninhabited. So while the comment is correct, it's kinda
redundant for the object -> bytes direction :)

No change required for this patch.

Best,
Gary

> +    (),
> +    {<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,
>