Use okref for mailbox packet struct

Sometimes when results are unwrapped, the compiler will memcopy the
Ok struct. okref will instead borrow the inner type.

Using this on the packet struct saves about 4KiB of stack space.

Author: jhand2

runtime/src/lib.rs

@@ -73,7 +73,7 @@ use tagging::{GetTaggedTciCmd, TagTciCmd};
 
 use caliptra_common::cprintln;
 
-use caliptra_drivers::{CaliptraError, CaliptraResult, ResetReason};
+use caliptra_drivers::{okmutref, CaliptraError, CaliptraResult, ResetReason};
 use caliptra_registers::el2_pic_ctrl::El2PicCtrl;
 use caliptra_registers::{mbox::enums::MboxStatusE, soc_ifc};
 use dpe::{
@@ -230,10 +230,11 @@ fn handle_command(drivers: &mut Drivers) -> CaliptraResult<MboxStatusE> {
         CommandId::AUTHORIZE_AND_STASH => AuthorizeAndStashCmd::execute(drivers, cmd_bytes),
         CommandId::GET_IDEV_CSR => GetIdevCsrCmd::execute(drivers, cmd_bytes),
         _ => Err(CaliptraError::RUNTIME_UNIMPLEMENTED_COMMAND),
-    }?;
+    };
+    let resp = okmutref(&mut resp)?;
 
     // Send the response
-    Packet::copy_to_mbox(drivers, &mut resp)?;
+    Packet::copy_to_mbox(drivers, resp)?;
 
     Ok(MboxStatusE::DataReady)
 }