From b8d0f59dcf7271167e8115b91b8210264e59568f Mon Sep 17 00:00:00 2001
From: Dylan Knutson <tcdknutson@gmail.com>
Date: Fri, 26 Jun 2026 00:29:37 +0000
Subject: [PATCH] uart-service: add default_mctp_serial constructor

Add MctpSerialService<R> and default_mctp_serial(), the MctpSerialMedium
twin of the existing DefaultService/default_smbusespi pair. The
constructor hardcodes the canonical EC->SP serial reply context
(source=EC_EID, destination=SP_EID, tag=0, medium_context=()), so serial
consumers (the QEMU EC<->SP relay path) call one constructor instead of
hand-assembling an MctpReplyContext. Enables mctp-rs/serial so the
MctpSerialMedium type is available.

Assisted-by: GitHub Copilot:claude-opus-4.8
---
 uart-service/Cargo.toml |  2 +-
 uart-service/src/lib.rs | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/uart-service/Cargo.toml b/uart-service/Cargo.toml
index beb373559..2d8fcb938 100644
--- a/uart-service/Cargo.toml
+++ b/uart-service/Cargo.toml
@@ -18,7 +18,7 @@ embedded-services.workspace = true
 defmt = { workspace = true, optional = true }
 log = { workspace = true, optional = true }
 embassy-sync.workspace = true
-mctp-rs = { workspace = true }
+mctp-rs = { workspace = true, features = ["serial"] }
 embedded-io-async.workspace = true
 
 [features]
diff --git a/uart-service/src/lib.rs b/uart-service/src/lib.rs
index e3ecffe6a..eb62e6402 100644
--- a/uart-service/src/lib.rs
+++ b/uart-service/src/lib.rs
@@ -188,3 +188,29 @@ impl<R: RelayHandler> DefaultService<R> {
         )
     }
 }
+
+/// Type alias for `MctpSerialMedium` services (DSP0253-style framed
+/// serial, no per-medium addressing). Used by the QEMU EC ↔ SP relay
+/// path where the secure PL011 is bridged via a host PTY.
+pub type MctpSerialService<R> = Service<R, mctp_rs::MctpSerialMedium>;
+
+impl<R: RelayHandler> MctpSerialService<R> {
+    /// Constructor for `MctpSerialMedium` services. Hardcodes the
+    /// EC ↔ SP reply context (`source = EC_EID`, `message_tag = 0`,
+    /// `medium_context = ()`). The `destination_endpoint_id` is
+    /// overridden per-response inside `process_response`, so the
+    /// `SP_EID` passed here is a placeholder.
+    pub fn default_mctp_serial(relay_handler: R) -> Result<Self, Error<mctp_rs::MctpSerialMedium>> {
+        Self::new(
+            relay_handler,
+            mctp_rs::MctpSerialMedium,
+            mctp_rs::MctpReplyContext {
+                source_endpoint_id: mctp_rs::EC_EID,
+                destination_endpoint_id: mctp_rs::SP_EID,
+                packet_sequence_number: mctp_rs::MctpSequenceNumber::new(0),
+                message_tag: mctp_rs::MctpMessageTag::try_from(0).map_err(Error::Serialize)?,
+                medium_context: (),
+            },
+        )
+    }
+}