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+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/arm-smc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ARM SMC Mailbox Interface
+
+maintainers:
+ - Peng Fan <peng.fan@nxp.com>
+
+description: |
+ This mailbox uses the ARM smc (secure monitor call) or hvc (hypervisor
+ call) instruction to trigger a mailbox-connected activity in firmware,
+ executing on the very same core as the caller. The value of r0/w0/x0
+ the firmware returns after the smc call is delivered as a received
+ message to the mailbox framework, so synchronous communication can be
+ established. The exact meaning of the action the mailbox triggers as
+ well as the return value is defined by their users and is not subject
+ to this binding.
+
+ One example use case of this mailbox is the SCMI interface, which uses
+ shared memory to transfer commands and parameters, and a mailbox to
+ trigger a function call. This allows SoCs without a separate management
+ processor (or when such a processor is not available or used) to use
+ this standardized interface anyway.
+
+ This binding describes no hardware, but establishes a firmware interface.
+ Upon receiving an SMC using the described SMC function identifier, the
+ firmware is expected to trigger some mailbox connected functionality.
+ The communication follows the ARM SMC calling convention.
+ Firmware expects an SMC function identifier in r0 or w0. The supported
+ identifier are passed from consumers, or listed in the the arm,func-id
+ property as described below. The firmware can return one value in
+ the first SMC result register, it is expected to be an error value,
+ which shall be propagated to the mailbox client.
+
+ Any core which supports the SMC or HVC instruction can be used, as long
+ as a firmware component running in EL3 or EL2 is handling these calls.
+
+properties:
+ compatible:
+ oneOf:
+ - description:
+ For implementations using ARM SMC instruction.
+ const: arm,smc-mbox
+
+ - description:
+ For implementations using ARM HVC instruction.
+ const: arm,hvc-mbox
+
+ "#mbox-cells":
+ const: 0
+
+ arm,func-id:
+ description: |
+ An single 32-bit value specifying the function ID used by the mailbox.
+ The function ID follows the ARM SMC calling convention standard.
+ $ref: /schemas/types.yaml#/definitions/uint32
+
+required:
+ - compatible
+ - "#mbox-cells"
+
+examples:
+ - |
+ sram@93f000 {
+ compatible = "mmio-sram";
+ reg = <0x0 0x93f000 0x0 0x1000>;
+ #address-cells = <1>;
+ #size-cells = <1>;
+ ranges = <0x0 0x93f000 0x1000>;
+
+ cpu_scp_lpri: scp-shmem@0 {
+ compatible = "arm,scmi-shmem";
+ reg = <0x0 0x200>;
+ };
+ };
+
+ smc_tx_mbox: tx_mbox {
+ #mbox-cells = <0>;
+ compatible = "arm,smc-mbox";
+ arm,func-id = <0xc20000fe>;
+ };
+
+ firmware {
+ scmi {
+ compatible = "arm,scmi";
+ mboxes = <&smc_tx_mbox>;
+ mbox-names = "tx";
+ shmem = <&cpu_scp_lpri>;
+ };
+ };
+
+...