Re: [PATCH V3] dt-bindings: mailbox: arm,mhuv2: Add bindings

This patch adds device tree binding for ARM Message Handling Unit (MHU)
controller version 2.

Based on earlier work by Morten Borup Petersen.

Co-developed-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Signed-off-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

---
V2->V3:

- compatible is changed to arm-mhuv2-tx/rx. Later version of MHUv2
  have an interrupt in the sender mode as well and we can't find if a
  mailbox is sender or receiver just based on an interrupt anymore. We
  needed a better way.

- arm-mhuv2-mode is renamed to arm,mhuv2-protocols to fit the purpose
  better.

- Minor formatting otherwise.

- No longer an RFC.

Morten: Please let me know if you want me to add your sign-off here from
some other email id as you no longer work with ARM.
---
 .../bindings/mailbox/arm,mhuv2.yaml           | 194 ++++++++++++++++++
 1 file changed, 194 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml

diff --git a/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml
new file mode 100644
index 000000000000..a2b8ecd639c7
--- /dev/null
+++ b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml

@@ -0,0 +1,194 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/arm,mhuv2.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ARM MHUv2 Mailbox Controller
+
+maintainers:
+  - Tushar Khandelwal <tushar.khandelwal@arm.com>
+  - Viresh Kumar <viresh.kumar@linaro.org>
+
+description: |
+  The Arm Message Handling Unit (MHU) Version 2 is a mailbox controller that has
+  between 1 and 124 channel windows (each 32-bit wide) to provide unidirectional
+  communication with remote processor(s), where the number of channel windows
+  are implementation dependent.
+
+  Given the unidirectional nature of the controller, an MHUv2 mailbox may only
+  be written to or read from. If a pair of MHU controllers is implemented
+  between two processing elements to provide bidirectional communication, these
+  must be specified as two separate mailboxes.
+
+  If the interrupts property is present in device tree node, then its treated as
+  a "receiver" mailbox, otherwise a "sender".
+
+  An MHU controller must be specified along with the supported transport
+  protocols. The transport protocols determine the method of data transmission
+  as well as the number of provided mailbox channels.
+
+  Following are the possible transport protocols.
+
+  - Doorbell: Each transfer is made up of single bit flag, using any one of the
+    bits in a channel window. A channel window can support up to 32 doorbells
+    and the entire window shall be used in doorbell protocol.  Optionally, data
+    may be transmitted through a shared memory region, wherein the MHU is used
+    strictly as an interrupt generation mechanism but that is out of the scope
+    of these bindings.
+
+  - Single-word: Each transfer is single word, using a single Channel window.
+
+  - Multi-word: Each transfer is made of two or more words, using two or more
+    channel windows.
+
+# We need a select here so we don't match all nodes with 'arm,primecell'
+select:
+  properties:
+    compatible:
+      contains:
+        enum:
+          - arm,mhuv2-tx
+          - arm,mhuv2-rx
+  required:
+    - compatible
+
+properties:
+  compatible:
+    oneOf:
+      - description: Sender mode
+        items:
+          - const: arm,mhuv2-tx
+          - const: arm,primecell
+
+      - description: Receiver-mode
+        items:
+          - const: arm,mhuv2-rx
+          - const: arm,primecell
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    description: |
+      The MHUv2 controller always implements an interrupt in the "receiver"
+      mode, while the interrupt in the "sender" mode was not available in the
+      version MHUv2.0, but the later versions do have it.
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    maxItems: 1
+
+  arm,mhuv2-protocols:
+    $ref: /schemas/types.yaml#/definitions/uint32-matrix
+    description: |
+      The MHUv2 controller may contain up to 124 channel windows (each 32-bit
+      wide). The hardware and the DT bindings allows any combination of those to
+      be used for various transport protocols.
+
+      This property allows a platform to describe how these channel windows are
+      used in various transport protocols. The entries in this property shall be
+      present as an array of tuples, where each tuple describes details about
+      one of the transport protocol being implemented over some channel
+      window(s).
+
+      The first field of a tuple signifies the transfer protocol, 0 is reserved
+      for doorbell protocol, 1 is reserved for single-word protocol and 2 is
+      reserved for multi-word protocol. Using any other value in the first field
+      of a tuple makes it invalid.
+
+      The second field of a tuple signifies the number of channel windows where
+      the protocol would be used. For doorbell protocol this field signifies the
+      number of 32-bit channel windows that implement the doorbell protocol. For
+      single-word protocol this field signifies the number of 32-bit channel
+      windows that implement separate single-word protocol mailbox channels. For
+      multi-word protocol this field signifies the number of channel windows
+      used for a multi-word protocol, it should be 2 or more.
+
+      The total number of channel windows specified here shouldn't be more than
+      the ones implemented by the platform, though one can specify lesser number
+      of windows here than what the platform implements.
+
+      mhu: mailbox@2b1f0000 {
+          ...
+
+          arm,mhuv2-protocols = <0 2>, <1 3>, <2 5>, <2 7>;
+      }
+
+      The above example defines the protocols of an ARM MHUv2 mailbox
+      controller, where a total of 17 channel windows are used. The first two
+      windows are used in doorbell protocol (64 doorbells), the next 3 windows
+      are (separately) used in single-word protocol, and the last two mailbox
+      channels are used in multi-word protocol of length 5 and 7 channel
+      windows.