Message-ID: <CAL_JsqKr9maL9W_9hQapPKFa+6S6wTr7WubHoHWtENyk74BDFA@mail.gmail.com>
Date: Mon, 25 Mar 2024 08:44:24 -0500
From: Rob Herring <robh+dt@...nel.org>
To: Cristian Marussi <cristian.marussi@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	devicetree@...r.kernel.org, sudeep.holla@....com, jassisinghbrar@...il.com, 
	krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org
Subject: Re: [PATCH 1/2] dt-bindings: mailbox: arm,mhuv3: Add bindings

On Mon, Mar 25, 2024 at 4:28 AM Cristian Marussi
<cristian.marussi@....com> wrote:
>
> Add bindings for the ARM MHUv3 Mailbox controller.
>
> Signed-off-by: Cristian Marussi <cristian.marussi@....com>
> ---
>  .../bindings/mailbox/arm,mhuv3.yaml           | 239 ++++++++++++++++++
>  1 file changed, 239 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/mailbox/arm,mhuv3.yaml
>
> diff --git a/Documentation/devicetree/bindings/mailbox/arm,mhuv3.yaml b/Documentation/devicetree/bindings/mailbox/arm,mhuv3.yaml
> new file mode 100644
> index 000000000000..bfb8ec341d26
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/mailbox/arm,mhuv3.yaml
> @@ -0,0 +1,239 @@
> +# 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@....com>
> +  - Cristian Marussi <cristian.marussi@....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 implemented is between 1 and
> +    128, numbered starting from 0 in ascending order.
> +    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 FCH that an implementation of the MHU
> +    can support is between 1-1024, if the Fast Channel word-size is 32-bits, or
> +    between 1-512, when the Fast Channel word-size is 64-bits.

Be consistent: FastChannel

> +    FCHs are numbered from 0 in ascending order.
> +    Note that Fast Channel word-size is 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 Transfer to the Receiver without having to wait for a 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 FFCH an implementation of the MHU can
> +    support is between 1 and 64, numbered starting from the 0 in ascending
> +    order. The FIFO size is implementation defined and not configurable.

Size is discoverable or opaque to s/w?

> +    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: 38
> +
> +  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, for sake of simplicity, only a limited number of
> +      per-channel interrupt names are enumerated, out of all the possible
> +      channels allowed by the specification for each extension type.
> +
> +      - "combined": PBX/MBX Combined interrupt
> +      - "combined-ffch": PBX/MBX FIFO Combined interrupt
> +      - "ffch-low-tide-<N>": PBX/MBX FIFO Channel <N> Low Tide interrupt
> +      - "ffch-high-tide-<N>": PBX/MBX FIFO Channel <N> High Tide interrupt
> +      - "ffch-flush-<N>": PBX/MBX FIFO Channel <N> Flush interrupt
> +      - "mbx-dbch-xfer-<N>": MBX Doorbell Channel <N> Transfer interrupt
> +      - "mbx-fch-xfer-<N>": MBX Fast Channel <N> Transfer interrupt
> +      - "mbx-fchgrp-xfer-<N>": MBX Fast Channel <N> Group Transfer interrupt
> +      - "mbx-ffch-xfer-<N>": MBX FIFO Channel <N> Transfer interrupt
> +      - "pbx-dbch-xfer-ack-<N>": PBX Doorbell Channel <N> Transfer Ack interrupt
> +      - "pbx-ffch-xfer-ack-<N>": PBX FIFO Channel <N> Transfer Ack interrupt

It would be better to not define these strings twice. You can do
something like this:

items:
  oneOf:
     - const: combined
       description: ...
     - pattern: '^ffch-low-tide-[0-9]+$'
       description: ...

And so on.

> +
> +      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: 38

Perhaps a comment on where 38 comes from. AIUI, this is a "should be
enough for now" value.

> +    items:
> +      enum:
> +        - combined
> +        - combined-ffch
> +        - ffch-low-tide-0
> +        - ffch-low-tide-1
> +        - ffch-low-tide-2
> +        - ffch-low-tide-3
> +        - ffch-high-tide-0
> +        - ffch-high-tide-1
> +        - ffch-high-tide-2
> +        - ffch-high-tide-3
> +        - ffch-flush-0
> +        - ffch-flush-1
> +        - ffch-flush-2
> +        - ffch-flush-3
> +        - mbx-dbch-xfer-0
> +        - mbx-dbch-xfer-1
> +        - mbx-dbch-xfer-2
> +        - mbx-dbch-xfer-3
> +        - mbx-fch-xfer-0
> +        - mbx-fch-xfer-1
> +        - mbx-fch-xfer-2
> +        - mbx-fch-xfer-3
> +        - mbx-fchgrp-xfer-0
> +        - mbx-fchgrp-xfer-1
> +        - mbx-fchgrp-xfer-2
> +        - mbx-fchgrp-xfer-3
> +        - mbx-ffch-xfer-0
> +        - mbx-ffch-xfer-1
> +        - mbx-ffch-xfer-2
> +        - mbx-ffch-xfer-3
> +        - pbx-dbch-xfer-ack-0
> +        - pbx-dbch-xfer-ack-1
> +        - pbx-dbch-xfer-ack-2
> +        - pbx-dbch-xfer-ack-3
> +        - pbx-ffch-xfer-ack-0
> +        - pbx-ffch-xfer-ack-1
> +        - pbx-ffch-xfer-ack-2
> +        - pbx-ffch-xfer-ack-3
> +
> +  '#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, Fast Channel Window 0.
> +      mboxes = <&mhu 1 3 0>; // FCE, Fast Channel 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>;
> +
> +        mhu_tx: mailbox@...a0000 {

Drop unused labels.

> +            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>;
> +        };
> +
> +        mhu_rx: mailbox@...00000 {
> +            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>;
> +        };
> +    };
> --
> 2.34.1
>

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