Re: [PATCH v2 4/6] gpu: nova-core: gsp: cmdq: move message trimming into its own function

["Eliot Courtney" <ecourtney@nvidia.com>]

On Fri Nov 6, 2026 at 4:06 PM JST, Alexandre Courbot wrote:
> Trimming the two driver-owned slices of the command queue to the
> advertised length of a message is going to be used by both RPC and GMC
> messages, so factor that part out into a private function.
>
> The parsing itself can be improved by leveraging `split_at_checked` and
> `get`, so use them.
>
> No functional change intended.
>
> Suggested-by: Eliot Courtney <ecourtney@nvidia.com>
> Link: https://lore.kernel.org/all/DMN62G2CDG3N.ETT4GRFXWKKU@nvidia.com/
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
>  drivers/gpu/nova-core/gsp/cmdq.rs     | 21 +++++++++++++++++++++
>  drivers/gpu/nova-core/gsp/cmdq/rpc.rs | 20 +-------------------
>  2 files changed, 22 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
> index 9dd11ac8952b..f3151aa8ea26 100644
> --- a/drivers/gpu/nova-core/gsp/cmdq.rs
> +++ b/drivers/gpu/nova-core/gsp/cmdq.rs
> @@ -719,3 +719,24 @@ fn drain(&mut self) -> Result {
>          }
>      }
>  }
> +
> +/// Helper function that trims the concatenation of `slice_1` and `slice_2` to `length`.
> +///
> +/// Returns the truncated `(slice_1, slice_2)` pair, or `EIO` if `length > slice_1.len() +
> +/// slice_2.len()`.
> +fn trim_message_slice<'a>(
> +    (slice_1, slice_2): (&'a [u8], &'a [u8]),
> +    length: usize,
> +) -> Result<(&'a [u8], &'a [u8])> {
> +    // See if `slice_1` covers the whole message, take it entirely if it doesn't.
> +    let slice_1 = slice_1
> +        .split_at_checked(length)
> +        .map(|split| split.0)
> +        .unwrap_or(slice_1);
> +    // `slice_2` is the remainder of the message not covered by `slice_1`.
> +    let slice_2 = slice_2
> +        .get(..length.saturating_sub(slice_1.len()))
> +        .ok_or(EIO)?;
> +
> +    Ok((slice_1, slice_2))
> +}

I think .split_at_checked(length).map(|x| x.0) is the same as
.get(..length), so you can prolly just write:

```
let slice_1 = slice_1.get(..length).unwrap_or(slice_1);
let slice_2 = slice_2.get(..length - slice_1.len()).ok_or(EIO)?;
```

Up to you if you want to use a checked_sub or saturating_sub but it's
locally clear that it won't underflow imo.

Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>