Re: [RFC V2] dt-bindings: mailbox : arm,mhuv2: Add bindings

From: Rob Herring
Date: Thu Oct 08 2020 - 10:31:14 EST


On Wed, Oct 7, 2020 at 10:28 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> 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: Morten Borup Petersen <morten.petersen@xxxxxxx>
> Signed-off-by: Morten Borup Petersen <morten.petersen@xxxxxxx>
> Signed-off-by: Tushar Khandelwal <tushar.khandelwal@xxxxxxx>
> Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
>
> ---
> Morten/Tushar,
>
> I have prepared this based on the best of my understanding about the
> hardware, but since I haven't worked on the actual hardware yet there
> are chances that I may have misunderstood some of it. Please lemme know
> if any of my understanding is incorrect.
>
> Here is the link to the previous attempt by Tushar to get it upstreamed
> long back:
>
> https://lore.kernel.org/lkml/20190717192616.1731-1-tushar.khandelwal@xxxxxxx/
>
> This is very much a new approach and so I am not pointing out the
> differences from the previous one.
>
> I haven't started with redesigning the driver as of now and would like
> to get some feedback on the bindings first. Thanks in advance.
> ---
> .../bindings/mailbox/arm,mhuv2.yaml | 170 ++++++++++++++++++
> 1 file changed, 170 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..3cbdc97f1f4c
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml
> @@ -0,0 +1,170 @@
> +# 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:
> + - Morten Borup Petersen <morten.petersen@xxxxxxx>
> + - Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> +
> +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 mode. 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:
> + const: arm,mhuv2
> + required:
> + - compatible
> +
> +properties:
> + compatible:
> + items:
> + - const: arm,mhuv2
> + - const: arm,primecell
> +
> + reg:
> + maxItems: 1
> +
> + interrupts:
> + maxItems: 1
> +
> + clocks:
> + maxItems: 1
> +
> + clock-names:
> + items:
> + - const: apb_pclk

Just 'maxItems: 1' is fine here as the name is already defined.

> +
> + arm-mhuv2-mode:

arm,mhuv2-mode.

Needs a type reference.

> + 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.

These are based on IP configuration or a software decision?

> + The total number of channel windows specified here shouldn't be more than
> + the ones implemented by the platform.

But can be less? Then do you need this to be a mask or range?

> +
> + mhu: mailbox@2b1f0000 {
> + ...
> +
> + arm-mhuv2-mode = <0 2>, <1 3>, <2 5>, <2 7>;
> + }
> +
> + The above example defines the mode of an ARM MHUv2 mailbox controller,
> + where a total of 17 channel windows are used. The first two windows are
> + used in doorbell mode (64 doorbells), the next 3 windows are (separately)
> + used in single-word mode, and the last two mailbox channels are used in
> + multi-word mode of length 5 and 7 channel windows.
> +
> + '#mbox-cells':
> + description: |
> + It contains two fields, the first field represents the channel number,
> + which may be used in doorbell, single-word, or multi-word mode, and the
> + second field (only relevant in doorbell mode, ignored otherwise)
> + represents the doorbell number within the 32 bit wide channel window.
> +
> + From the example given above for the arm-mhuv2-mode, here is how a client
> + node can reference them.
> +
> + mboxes = <&mhu 0 5>; // Mailbox channel 0, doorbell 5.
> + mboxes = <&mhu 1 7>; // Mailbox channel 1, doorbell 7.
> + mboxes = <&mhu 2 0>; // Mailbox channel 2, single-word mode.
> + mboxes = <&mhu 4 0>; // Mailbox channel 4, single-word mode.
> + mboxes = <&mhu 6 0>; // Mailbox channel 6, multi-word mode with 7 windows.
> +
> + const: 2
> +
> +required:
> + - compatible
> + - reg
> + - '#mbox-cells'
> + - arm-mhuv2-mode
> +
> +additionalProperties: false
> +
> +examples:
> + # Multiple transport protocols implemented by the mailbox controller
> + # configured in sender mode
> + - |
> + soc {
> + #address-cells = <2>;
> + #size-cells = <2>;
> +
> + mhu: mailbox@2b1f0000 {
> + #mbox-cells = <2>;
> + compatible = "arm,mhuv2", "arm,primecell";
> + reg = <0 0x2b1f0000 0 0x1000>;
> + clocks = <&clock 0>;
> + clock-names = "apb_pclk";
> + arm-mhuv2-mode = <1 5>, <2 2>, <2 5>, <2 7>, <0 2>;
> + };
> +
> + mhu_client: scb@2e000000 {
> + compatible = "fujitsu,mb86s70-scb-1.0";
> + reg = <0 0x2e000000 0 0x4000>;
> +
> + mboxes =
> + //single-word protocol channel 0
> + <&mhu 0 0>,
> + //single-word protocol channel 4
> + <&mhu 4 0>,
> + //multi-word protocol channel 6 with 5 windows
> + <&mhu 6 0>,
> + //doorbell protocol channel 9, doorbell 27
> + <&mhu 9 27>;
> + };
> + };
> --
> 2.25.0.rc1.19.g042ed3e048af
>