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SubjectRe: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc for the ARM SMC/HVC mailbox
On Wed, Aug 28, 2019 at 03:02:58AM +0000, Peng Fan wrote:
> From: Peng Fan <>
> 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 <>
> ---
> .../devicetree/bindings/mailbox/arm-smc.yaml | 125 +++++++++++++++++++++
> 1 file changed, 125 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..f8eb28d5e307
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/mailbox/arm-smc.yaml
> @@ -0,0 +1,125 @@
> +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id:
> +$schema:
> +
> +title: ARM SMC Mailbox Interface
> +
> +maintainers:
> + - Peng Fan <>
> +
> +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.

What do you mean by that ? I would prefer to drop the above line unless
I am missing something. IMO it contradicts the previous statement less
you elaborate more on this.

> 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.

I assume you refer to asynchronous communication from OS to firmware in the
above statement and "not asynchronous notification" from firmware to OS.

> + 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.
> +

Not sure if reference to SCMI is needed at all but I don't have any
objections to it, just thought worth mentioning.

> + 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.
> +

Other than the above points, I am fine with it. Once fixed,

Reviewed-by: Sudeep Holla <>

Note I haven't reviewed the yaml scheme, but just binding in general.


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