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+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/arm,mhuv3.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ARM MHUv3 Mailbox Controller
+
+maintainers:
+  - Sudeep Holla <sudeep.holla@arm.com>
+  - Cristian Marussi <cristian.marussi@arm.com>
+
+description: |
+  The Arm Message Handling Unit (MHU) Version 3 is a mailbox controller that
+  enables unidirectional communications with remote processors through various
+  possible transport protocols.
+  The controller can optionally support a varying number of extensions that, in
+  turn, enable different kinds of transport to be used for communication.
+  Number, type and characteristics of each supported extension can be discovered
+  dynamically at runtime.
+
+  Given the unidirectional nature of the controller, an MHUv3 mailbox controller
+  is composed of a MHU Sender (MHUS) containing a PostBox (PBX) block and a MHU
+  Receiver (MHUR) containing a MailBox (MBX) block, where
+
+   PBX is used to
+      - Configure the MHU
+      - Send Transfers to the Receiver
+      - Optionally receive acknowledgment of a Transfer from the Receiver
+
+   MBX is used to
+      - Configure the MHU
+      - Receive Transfers from the Sender
+      - Optionally acknowledge Transfers sent by the Sender
+
+  Both PBX and MBX need to be present and defined in the DT description if you
+  need to establish a bidirectional communication, since you will have to
+  acquire two distinct unidirectional channels, one for each block.
+
+  As a consequence both blocks needs to be represented separately and specified
+  as distinct DT nodes in order to properly describe their resources.
+
+  Note that, though, thanks to the runtime discoverability, there is no need to
+  identify the type of blocks with distinct compatibles.
+
+  Following are the MHUv3 possible extensions.
+
+  - Doorbell Extension (DBE): DBE defines a type of channel called a Doorbell
+    Channel (DBCH). DBCH enables a single bit Transfer to be sent from the
+    Sender to Receiver. The Transfer indicates that an event has occurred.
+    When DBE is implemented, the number of DBCHs that an implementation of the
+    MHU can support is between 1 and 128, numbered starting from 0 in ascending
+    order and discoverable at run-time.
+    Each DBCH contains 32 individual fields, referred to as flags, each of which
+    can be used independently. It is possible for the Sender to send multiple
+    Transfers at once using a single DBCH, so long as each Transfer uses
+    a different flag in the DBCH.
+    Optionally, data may be transmitted through an out-of-band shared memory
+    region, wherein the MHU Doorbell is used strictly as an interrupt generation
+    mechanism, but this is out of the scope of these bindings.
+
+  - FastChannel Extension (FCE): FCE defines a type of channel called a Fast
+    Channel (FCH). FCH is intended for lower overhead communication between
+    Sender and Receiver at the expense of determinism. An FCH allows the Sender
+    to update the channel value at any time, regardless of whether the previous
+    value has been seen by the Receiver. When the Receiver reads the channel's
+    content it gets the last value written to the channel.
+    FCH is considered lossy in nature, and means that the Sender has no way of
+    knowing if, or when, the Receiver will act on the Transfer.
+    FCHs are expected to behave as RAM which generates interrupts when writes
+    occur to the locations within the RAM.
+    When FCE is implemented, the number of FCHs that an implementation of the
+    MHU can support is between 1-1024, if the FastChannel word-size is 32-bits,
+    or between 1-512, when the FastChannel word-size is 64-bits.
+    FCHs are numbered from 0 in ascending order.
+    Note that the number of FCHs and the word-size are implementation defined,
+    not configurable but discoverable at run-time.
+    Optionally, data may be transmitted through an out-of-band shared memory
+    region, wherein the MHU FastChannel is used as an interrupt generation
+    mechanism which carries also a pointer to such out-of-band data, but this
+    is out of the scope of these bindings.
+
+  - FIFO Extension (FE): FE defines a Channel type called a FIFO Channel (FFCH).
+    FFCH allows a Sender to send
+       - Multiple Transfers to the Receiver without having to wait for the
+         previous Transfer to be acknowledged by the Receiver, as long as the
+         FIFO has room for the Transfer.
+       - Transfers which require the Receiver to provide acknowledgment.
+       - Transfers which have in-band payload.
+    In all cases, the data is guaranteed to be observed by the Receiver in the
+    same order which the Sender sent it.
+    When FE is implemented, the number of FFCHs that an implementation of the
+    MHU can support is between 1 and 64, numbered starting from 0 in ascending
+    order. The number of FFCHs, their depth (same for all implemented FFCHs) and
+    the access-granularity are implementation defined, not configurable but
+    discoverable at run-time.
+    Optionally, additional data may be transmitted through an out-of-band shared
+    memory region, wherein the MHU FIFO is used to transmit, in order, a small
+    part of the payload (like a header) and a reference to the shared memory
+    area holding the remaining, bigger, chunk of the payload, but this is out of
+    the scope of these bindings.
+
+properties:
+  compatible:
+    const: arm,mhuv3
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    minItems: 1
+    maxItems: 74
+
+  interrupt-names:
+    description: |
+      The MHUv3 controller generates a number of events some of which are used
+      to generate interrupts; as a consequence it can expose a varying number of
+      optional PBX/MBX interrupts, representing the events generated during the
+      operation of the various transport protocols associated with different
+      extensions. All interrupts of the MHU are level-sensitive.
+      Some of these optional interrupts are defined per-channel, where the
+      number of channels effectively available is implementation defined and
+      run-time discoverable.
+      In the following names are enumerated using patterns, with per-channel
+      interrupts implicitly capped at the maximum channels allowed by the
+      specification for each extension type.
+      For the sake of simplicity maxItems is anyway capped to a most plausible
+      number, assuming way less channels would be implemented than actually
+      possible.
+
+      The only mandatory interrupts on the MHU are:
+        - combined
+        - mbx-fch-xfer-<N> but only if mbx-fcgrp-xfer-<N> is not implemented.
+
+    minItems: 1
+    maxItems: 74
+    items:
+      oneOf:
+        - const: combined
+          description: PBX/MBX Combined interrupt
+        - const: combined-ffch
+          description: PBX/MBX FIFO Combined interrupt
+        - pattern: '^ffch-low-tide-[0-9]+$'
+          description: PBX/MBX FIFO Channel <N> Low Tide interrupt
+        - pattern: '^ffch-high-tide-[0-9]+$'
+          description: PBX/MBX FIFO Channel <N> High Tide interrupt
+        - pattern: '^ffch-flush-[0-9]+$'
+          description: PBX/MBX FIFO Channel <N> Flush interrupt
+        - pattern: '^mbx-dbch-xfer-[0-9]+$'
+          description: MBX Doorbell Channel <N> Transfer interrupt
+        - pattern: '^mbx-fch-xfer-[0-9]+$'
+          description: MBX FastChannel <N> Transfer interrupt
+        - pattern: '^mbx-fchgrp-xfer-[0-9]+$'
+          description: MBX FastChannel <N> Group Transfer interrupt
+        - pattern: '^mbx-ffch-xfer-[0-9]+$'
+          description: MBX FIFO Channel <N> Transfer interrupt
+        - pattern: '^pbx-dbch-xfer-ack-[0-9]+$'
+          description: PBX Doorbell Channel <N> Transfer Ack interrupt
+        - pattern: '^pbx-ffch-xfer-ack-[0-9]+$'
+          description: PBX FIFO Channel <N> Transfer Ack interrupt
+
+  '#mbox-cells':
+    description: |
+      The first argument in the consumers 'mboxes' property represents the
+      extension type, the second is for the channel number while the third
+      depends on extension type.
+
+      Extension type for DBE is 0 and the third parameter represents the
+      doorbell flag number to use.
+      Extension type for FCE is 1, third parameter unused.
+      Extension type for FE is 2, third parameter unused.
+
+      mboxes = <&mhu 0 0 5>; // DBE, Doorbell Channel Window 0, doorbell flag 5.
+      mboxes = <&mhu 0 1 7>; // DBE, Doorbell Channel Window 1, doorbell flag 7.
+      mboxes = <&mhu 1 0 0>; // FCE, FastChannel Window 0.
+      mboxes = <&mhu 1 3 0>; // FCE, FastChannel Window 3.
+      mboxes = <&mhu 2 1 0>; // FE, FIFO Channel Window 1.
+      mboxes = <&mhu 2 7 0>; // FE, FIFO Channel Window 7.
+    const: 3
+
+  clocks:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - interrupt-names
+  - '#mbox-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    soc {
+        #address-cells = <2>;
+        #size-cells = <2>;
+
+        mailbox@2aaa0000 {
+            compatible = "arm,mhuv3";
+            #mbox-cells = <3>;
+            reg = <0 0x2aaa0000 0 0x10000>;
+            clocks = <&clock 0>;
+            interrupt-names = "combined", "pbx-dbch-xfer-ack-1",
+                               "ffch-high-tide-0";
+            interrupts = <0 36 4>, <0 37 4>;
+        };
+
+        mailbox@2ab00000 {
+            compatible = "arm,mhuv3";
+            #mbox-cells = <3>;
+            reg = <0 0x2aab0000 0 0x10000>;
+            clocks = <&clock 0>;
+            interrupt-names = "combined", "mbx-dbch-xfer-1", "ffch-low-tide-0";
+            interrupts = <0 35 4>, <0 38 4>, <0 39 4>;
+        };
+    };
