Subject: Re: [PATCH V9 1/2] dt-bindings: mailbox: add binding doc for the ARM SMC/HVC mailbox

From: Andre Przywara <[email-masked]>
Date: Wed, 25 Sep 2019 18:09:01 +0100

On Wed, 25 Sep 2019 02:09:08 +0000
Peng Fan <[email-masked]> wrote:

Hi,

> From: Peng Fan <[email-masked]>
> 
> 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 <[email-masked]>
> Reviewed-by: Florian Fainelli <[email-masked]>
> ---
>  .../devicetree/bindings/mailbox/arm-smc.yaml       | 96 ++++++++++++++++++++++
>  1 file changed, 96 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..b061954d1678
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/mailbox/arm-smc.yaml
> @@ -0,0 +1,96 @@
> +# 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 <[email-masked]>
> +
> +description: |
> +  This mailbox uses the ARM smc (secure monitor call) or hvc (hypervisor
> +  call) instruction to trigger a mailbox-connected activity in firmware,
> +  executing on the very same core as the caller. The value of r0/w0/x0
> +  the firmware returns after the smc call is delivered as a received
> +  message to the mailbox framework, so synchronous communication can be
> +  established. The exact meaning of the action the mailbox triggers as
> +  well as the return value is defined by their users and is not subject
> +  to this binding.
> +
> +  One example 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 the described SMC function identifier, 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
> +  identifier are passed from consumers, or listed in the the arm,func-id

                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
		This is now obsolete.

The rest looks good to me, thanks for the changes!

Reviewed-by: Andre Przywara <[email-masked]>

Cheers,
Andre.

> +  property 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:
> +    oneOf:
> +      - description:
> +          For implementations using ARM SMC instruction.
> +        const: arm,smc-mbox
> +
> +      - description:
> +          For implementations using ARM HVC instruction.
> +        const: arm,hvc-mbox
> +
> +  "#mbox-cells":
> +    const: 0
> +
> +  arm,func-id:
> +    description: |
> +      An single 32-bit value specifying the function ID used by the mailbox.
> +      The function ID follows the ARM SMC calling convention standard.
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +
> +required:
> +  - compatible
> +  - "#mbox-cells"
> +  - arm,func-id
> +
> +examples:
> +  - |
> +    sram@93f000 {
> +      compatible = "mmio-sram";
> +      reg = <0x0 0x93f000 0x0 0x1000>;
> +      #address-cells = <1>;
> +      #size-cells = <1>;
> +      ranges = <0x0 0x93f000 0x1000>;
> +
> +      cpu_scp_lpri: scp-shmem@0 {
> +        compatible = "arm,scmi-shmem";
> +        reg = <0x0 0x200>;
> +      };
> +    };
> +
> +    smc_tx_mbox: tx_mbox {
> +      #mbox-cells = <0>;
> +      compatible = "arm,smc-mbox";
> +      arm,func-id = <0xc20000fe>;
> +    };
> +
> +    firmware {
> +      scmi {
> +        compatible = "arm,scmi";
> +        mboxes = <&smc_tx_mbox>;
> +        mbox-names = "tx";
> +        shmem = <&cpu_scp_lpri>;
> +      };
> +    };
> +
> +...

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