gpu: nova-core: gsp: fix length of received messages

The size of messages' payload is miscalculated, leading to extra data
passed to the message handler. While this is not a problem with our
current set of commands, others with a variable-length payload may
misbehave. Fix this by introducing a method returning the payload size
and using it.

Fixes: 75f6b1de81 ("gpu: nova-core: gsp: Add GSP command queue bindings and handling")
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20251216-nova-fixes-v3-2-c7469a71f7c4@nvidia.com
[acourbot@nvidia.com: update `PANIC:` comments as pointed out by Joel.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

drivers/gpu/nova-core/gsp/fw.rs

@@ -853,11 +853,16 @@ impl GspMsgElement {
         self.inner.checkSum = checksum;
     }
 
-    /// Returns the total length of the message.
+    /// Returns the length of the message's payload.
+    pub(crate) fn payload_length(&self) -> usize {
+        // `rpc.length` includes the length of the RPC message header.
+        num::u32_as_usize(self.inner.rpc.length)
+            .saturating_sub(size_of::<bindings::rpc_message_header_v>())
+    }
+
+    /// Returns the total length of the message, message and RPC headers included.
     pub(crate) fn length(&self) -> usize {
-        // `rpc.length` includes the length of the GspRpcHeader but not the message header.
-        size_of::<Self>() - size_of::<bindings::rpc_message_header_v>()
-            + num::u32_as_usize(self.inner.rpc.length)
+        size_of::<Self>() + self.payload_length()
     }
 
     // Returns the sequence number of the message.