diff --git a/Documentation/devicetree/bindings/mailbox/nvidia,tegra186-hsp.yaml b/Documentation/devicetree/bindings/mailbox/nvidia,tegra186-hsp.yaml
index 9f7a7296b57ff875abb09a9cd61f905548e82f57..a3e87516d637d8f9596bd41b68f10db543ea97ff 100644
--- a/Documentation/devicetree/bindings/mailbox/nvidia,tegra186-hsp.yaml
+++ b/Documentation/devicetree/bindings/mailbox/nvidia,tegra186-hsp.yaml
@@ -26,6 +26,15 @@ description: |
   second cell is used to identify the mailbox that the client is going
   to use.
 
+  For shared mailboxes, the first cell composed of two fields:
+    - bits 15..8:
+        A bit mask of flags that further specifies the type of shared
+        mailbox to be used (based on the data size). If no flag is
+        specified then, 32-bit shared mailbox is used.
+    - bits 7..0:
+        Defines the type of the mailbox to be used. This field should be
+        TEGRA_HSP_MBOX_TYPE_SM for shared mailboxes.
+
   For doorbells, the second cell specifies the index of the doorbell to
   use.
 
diff --git a/include/dt-bindings/mailbox/tegra186-hsp.h b/include/dt-bindings/mailbox/tegra186-hsp.h
index 3bdec7a84d35cfd98c2d203aff7b11792ca3e4f4..b9ccae2aa9e299040c0d931acb89709a113d4f58 100644
--- a/include/dt-bindings/mailbox/tegra186-hsp.h
+++ b/include/dt-bindings/mailbox/tegra186-hsp.h
@@ -15,6 +15,11 @@
 #define TEGRA_HSP_MBOX_TYPE_SS 0x2
 #define TEGRA_HSP_MBOX_TYPE_AS 0x3
 
+/*
+ * These define the types of shared mailbox supported based on data size.
+ */
+#define TEGRA_HSP_MBOX_TYPE_SM_128BIT (1 << 8)
+
 /*
  * These defines represent the bit associated with the given master ID in the
  * doorbell registers.