Re: [PATCH v3 1/2] dt-bindings: mailbox: add binding doc for the ARM SMC/HVC mailbox

[Sudeep Holla <sudeep.holla@arm.com>]

This looks much better now.

On Mon, Jul 15, 2019 at 10:10:10AM +0000, Peng Fan wrote:
> From: Peng Fan <peng.fan@nxp.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@nxp.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@nxp.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
> +
> +  method:
> +    items:
> +      - enum:
> +          - smc
> +          - hvc
> +
> +  transports:
> +    items:
> +      - enum:
> +          - mem
> +          - reg
> +
> +  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].
> +
> +      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?

I am new to yaml, is there a way to say the number of entries here must
match arm,num-chans ? And not sure if min/maxItems matter then ?

> +
> +required:
> +  - compatible
> +  - "#mbox-cells"
> +  - arm,num-chans
> +  - transports
> +  - method
> +

Why is arm,func-ids optional ? Is there any standard arm,func-ids we can
resort to. Sorry I know you expect ARM Ltd to answer that, but I just want
to raise the point that we don't have one today and hence it can't be
optional. Or I am missing something ?

--
Regards,
Sudeep

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