intercreate · JPHutchins · May 29, 2026
diff --git a/boot/boot_serial/src/boot_serial.c b/boot/boot_serial/src/boot_serial.c
@@ -1261,6 +1261,34 @@ bs_echo(char *buf, int len)
 }
 #endif
 
+#ifdef MCUBOOT_BOOT_MGMT_MCUMGR_PARAMS
+/*
+ * Reports the SMP transport buffer parameters so that clients can negotiate
+ * optimal serial fragmentation, mirroring the mcumgr OS group "MCUmgr
+ * parameters" command provided by Zephyr's SMP server. The serial recovery
+ * reassembles one command at a time into a single buffer, hence a buffer
+ * count of 1. The line length is not reported; enabling this command requires
+ * BOOT_MAX_LINE_INPUT_LEN to remain at the standard 128-byte fragment size.
+ */
+static void
+bs_mcumgr_params(char *buf, int len)
+{
+    if (
+        zcbor_map_start_encode(cbor_state, 10) &&
+        zcbor_tstr_put_lit_cast(cbor_state, "buf_size") &&
+        zcbor_uint32_put(cbor_state, MCUBOOT_SERIAL_MAX_RECEIVE_SIZE) &&
+        zcbor_tstr_put_lit_cast(cbor_state, "buf_count") &&
+        zcbor_uint32_put(cbor_state, 1) &&
+        zcbor_map_end_encode(cbor_state, 10)
+    ) {
+        boot_serial_output();
+    } else {
+        reset_cbor_state();
+        bs_rc_rsp(MGMT_ERR_ENOMEM);
+    }
+}
+#endif
+
 /*
  * Reset, and (presumably) boot to newly uploaded image. Flush console
  * before restarting.
@@ -1354,6 +1382,11 @@ boot_serial_input(char *buf, int len)
         case NMGR_ID_RESET:
             bs_reset(buf, len);
             break;
+#ifdef MCUBOOT_BOOT_MGMT_MCUMGR_PARAMS
+        case NMGR_ID_MCUMGR_PARAMS:
+            bs_mcumgr_params(buf, len);
+            break;
+#endif
         default:
             bs_rc_rsp(MGMT_ERR_ENOTSUP);
             break;

diff --git a/boot/boot_serial/src/boot_serial_priv.h b/boot/boot_serial/src/boot_serial_priv.h
@@ -54,6 +54,7 @@ extern "C" {
 #define NMGR_ID_ECHO            0
 #define NMGR_ID_CONS_ECHO_CTRL  1
 #define NMGR_ID_RESET           5
+#define NMGR_ID_MCUMGR_PARAMS   6
 
 #ifndef __packed
 #define __packed __attribute__((__packed__))

diff --git a/boot/zephyr/Kconfig.serial_recovery b/boot/zephyr/Kconfig.serial_recovery
@@ -111,6 +111,19 @@ config BOOT_MGMT_ECHO
 	help
 	  if enabled, support for the mcumgr echo command is being added.
 
+config BOOT_MGMT_MCUMGR_PARAMS
+	bool "SMP server buffer size"
+	depends on BOOT_MAX_LINE_INPUT_LEN = 128
+	help
+	  If enabled, support for the mcumgr OS group "MCUmgr parameters"
+	  command (command ID 6) is added, mirroring the command provided by
+	  Zephyr's SMP server. It reports the SMP transport buffer size
+	  (BOOT_SERIAL_MAX_RECEIVE_SIZE) and buffer count so that SMP clients
+	  can negotiate optimal serial fragmentation. The parameters do not
+	  carry the transport line length, so this option requires
+	  BOOT_MAX_LINE_INPUT_LEN to remain at the standard 128-byte fragment
+	  size. buf_count will always be 1.
+
 menuconfig ENABLE_MGMT_PERUSER
 	bool "System specific mcumgr commands"
 	help

diff --git a/boot/zephyr/include/mcuboot_config/mcuboot_config.h b/boot/zephyr/include/mcuboot_config/mcuboot_config.h
@@ -296,6 +296,10 @@
 #define MCUBOOT_BOOT_MGMT_ECHO
 #endif
 
+#ifdef CONFIG_BOOT_MGMT_MCUMGR_PARAMS
+#define MCUBOOT_BOOT_MGMT_MCUMGR_PARAMS
+#endif
+
 #ifdef CONFIG_BOOT_IMAGE_ACCESS_HOOKS
 #define MCUBOOT_IMAGE_ACCESS_HOOKS
 #endif

diff --git a/boot/zephyr/sample.yaml b/boot/zephyr/sample.yaml
@@ -54,6 +54,7 @@ tests:
       - CONFIG_ENABLE_MGMT_PERUSER=y
       - CONFIG_BOOT_MGMT_CUSTOM_STORAGE_ERASE=y
       - CONFIG_BOOT_MGMT_ECHO=y
+      - CONFIG_BOOT_MGMT_MCUMGR_PARAMS=y
       - CONFIG_BOOT_SERIAL_IMG_GRP_IMAGE_STATE=y
       - CONFIG_BOOT_SERIAL_IMG_GRP_SLOT_INFO=y
     platform_allow:  nrf52840dk/nrf52840

diff --git a/docs/release-notes.d/serial-recovery-mcumgr-params.md b/docs/release-notes.d/serial-recovery-mcumgr-params.md
@@ -0,0 +1,7 @@
+- Added an optional serial recovery ``MCUmgr parameters`` command (mcumgr OS
+  management group, command ID 6), enabled with
+  ``CONFIG_BOOT_MGMT_MCUMGR_PARAMS``. The read-only command reports the SMP
+  transport ``buf_size`` and ``buf_count`` so that SMP clients can negotiate
+  optimal serial fragmentation, matching the command provided by Zephyr's SMP
+  server. It requires ``CONFIG_BOOT_MAX_LINE_INPUT_LEN`` to remain at its
+  default of 128.
diff --git a/docs/serial_recovery.md b/docs/serial_recovery.md
@@ -32,6 +32,7 @@ See the Zephyr [Device Management](https://docs.zephyrproject.org/latest/service
 MCUboot supports the following subset of the MCUmgr commands:
 * echo (OS group)
 * reset (OS group)
+* MCUmgr parameters SMP buffer size (OS group), when ``MCUBOOT_BOOT_MGMT_MCUMGR_PARAMS`` is enabled
 * image list (IMG group)
 * image upload (IMG group)