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

[Alexandre Courbot <acourbot@nvidia.com>]

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))
+}
diff --git a/drivers/gpu/nova-core/gsp/cmdq/rpc.rs b/drivers/gpu/nova-core/gsp/cmdq/rpc.rs
index 36cba7c28d3e..6ee28e93586b 100644
--- a/drivers/gpu/nova-core/gsp/cmdq/rpc.rs
+++ b/drivers/gpu/nova-core/gsp/cmdq/rpc.rs
@@ -214,27 +214,9 @@ pub(super) fn parse(dev: &device::Device, element: GspMessage<'a>) -> Result<Rpc
             payload_length,
         );
 
-        // Check that the driver read area is large enough for the message.
-        if slice_1.len() + slice_2.len() < payload_length {
-            return Err(EIO);
-        }
-
-        // Cut the message slices down to the actual length of the message.
-        let (slice_1, slice_2) = if slice_1.len() > payload_length {
-            // PANIC: we checked above that `slice_1` is at least as long as `payload_length`.
-            (slice_1.split_at(payload_length).0, &slice_2[0..0])
-        } else {
-            (
-                slice_1,
-                // PANIC: we checked above that `slice_1.len() + slice_2.len()` is at least as
-                // large as `payload_length`.
-                slice_2.split_at(payload_length - slice_1.len()).0,
-            )
-        };
-
         Ok(RpcMessage {
             header: rpc_header,
-            contents: (slice_1, slice_2),
+            contents: super::trim_message_slice((slice_1, slice_2), payload_length)?,
         })
     }
 

-- 
2.56.0