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Date:   Mon, 15 Jul 2019 11:03:03 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Peng Fan <peng.fan@....com>
Cc:     "mark.rutland@....com" <mark.rutland@....com>,
        "jassisinghbrar@...il.com" <jassisinghbrar@...il.com>,
        "sudeep.holla@....com" <sudeep.holla@....com>,
        "andre.przywara@....com" <andre.przywara@....com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH v3 1/2] dt-bindings: mailbox: add binding doc for the ARM
 SMC/HVC mailbox

On Mon, Jul 15, 2019 at 4:10 AM Peng Fan <peng.fan@....com> wrote:
>
> From: Peng Fan <peng.fan@....com>
>
> The ARM SMC/HVC mailbox binding describes a firmware interface to trigger
> actions in software layers running in the EL2 or EL3 exception levels.
> The term "ARM" here relates to the SMC instruction as part of the ARM
> instruction set, not as a standard endorsed by ARM Ltd.
>
> Signed-off-by: Peng Fan <peng.fan@....com>
> ---
>
> V3:
>  Convert to yaml
>  Drop interrupt
>  Introudce transports to indicate mem/reg
>  The func id is still kept as optional, because like SCMI it only
>  cares about message.
>
> V2:
>  Introduce interrupts as a property.
>
>  .../devicetree/bindings/mailbox/arm-smc.yaml       | 124 +++++++++++++++++++++
>  1 file changed, 124 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/mailbox/arm-smc.yaml
>
> diff --git a/Documentation/devicetree/bindings/mailbox/arm-smc.yaml b/Documentation/devicetree/bindings/mailbox/arm-smc.yaml
> new file mode 100644
> index 000000000000..da9b1a03bc4e
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/mailbox/arm-smc.yaml
> @@ -0,0 +1,124 @@
> +# 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@....com>
> +
> +description: |
> +  This mailbox uses the ARM smc (secure monitor call) and hvc (hypervisor
> +  call) instruction to trigger a mailbox-connected activity in firmware,
> +  executing on the very same core as the caller. By nature this operation
> +  is synchronous and this mailbox provides no way for asynchronous messages
> +  to be delivered the other way round, from firmware to the OS, but
> +  asynchronous notification could also be supported. However the value of
> +  r0/w0/x0 the firmware returns after the smc call is delivered as a received
> +  message to the mailbox framework, so a synchronous communication can be
> +  established, for a asynchronous notification, no value will be returned.
> +  The exact meaning of both the action the mailbox triggers as well as the
> +  return value is defined by their users and is not subject to this binding.
> +
> +  One 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 one of the described SMC function identifiers,
> +  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
> +  identifiers are passed from consumers, or listed in the the arm,func-ids
> +  properties 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:
> +    const: arm,smc-mbox
> +
> +  "#mbox-cells":
> +    const: 1
> +
> +  arm,num-chans:
> +    description: The number of channels supported.
> +    $ref: /schemas/types.yaml#/definitions/uint32

Constraints? 0 is valid? 2^32?

> +
> +  method:
> +    items:
> +      - enum:
> +          - smc
> +          - hvc
> +
> +  transports:
> +    items:
> +      - enum:
> +          - mem
> +          - reg

What if someone wants to configure this per channel? Perhaps
#mbox-cells should be 2 and this can be a client parameter.

Minimally, this needs a 'arm' vendor prefix if it stays.

> +
> +  arm,func-ids:
> +    description: |
> +      An array of 32-bit values specifying the function IDs used by each
> +      mailbox channel. Those function IDs follow the ARM SMC calling
> +      convention standard [1].

What's the default if not specified? Or this should be required?

> +
> +      There is one identifier per channel and the number of supported
> +      channels is determined by the length of this array.
> +    minItems: 0
> +    maxItems: 4096   # Should be enough?
> +
> +required:
> +  - compatible
> +  - "#mbox-cells"
> +  - arm,num-chans
> +  - transports
> +  - method
> +
> +examples:
> +  - |
> +    sram@...000 {
> +      compatible = "mmio-sram";
> +      reg = <0x0 0x93f000 0x0 0x1000>;
> +      #address-cells = <1>;
> +      #size-cells = <1>;
> +      ranges = <0 0x0 0x93f000 0x1000>;
> +
> +        cpu_scp_lpri: scp-shmem@0 {
> +          compatible = "arm,scmi-shmem";
> +          reg = <0x0 0x200>;
> +        };
> +
> +        cpu_scp_hpri: scp-shmem@200 {
> +          compatible = "arm,scmi-shmem";
> +          reg = <0x200 0x200>;
> +        };
> +    };
> +
> +    firmware {
> +      smc_mbox: mailbox {
> +        #mbox-cells = <1>;
> +        compatible = "arm,smc-mbox";
> +        method = "smc";
> +        arm,num-chans = <0x2>;
> +        transports = "mem";
> +        /* Optional */
> +        arm,func-ids = <0xc20000fe>, <0xc20000ff>;
> +      };
> +
> +      scmi {
> +        compatible = "arm,scmi";
> +        mboxes = <&mailbox 0 &mailbox 1>;
> +        mbox-names = "tx", "rx";
> +        shmem = <&cpu_scp_lpri &cpu_scp_hpri>;
> +      };
> +    };
> +
> +...
> --
> 2.16.4
>

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