Re: [PATCH V2 1/2] DT: mailbox: add binding doc for the ARM SMC mailbox

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

On Mon, Jun 03, 2019 at 04:30:04PM +0800, peng.fan@nxp.com wrote:
> From: Peng Fan <peng.fan@nxp.com>
> 
> The ARM SMC 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>
> ---
> 
> V2:
> Introduce interrupts as a property.
> 
> V1:
> arm,func-ids is still kept as an optional property, because there is no
> defined SMC funciton id passed from SCMI. So in my test, I still use
> arm,func-ids for ARM SIP service.
> 
>  .../devicetree/bindings/mailbox/arm-smc.txt        | 101 +++++++++++++++++++++
>  1 file changed, 101 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/mailbox/arm-smc.txt
> 
> diff --git a/Documentation/devicetree/bindings/mailbox/arm-smc.txt b/Documentation/devicetree/bindings/mailbox/arm-smc.txt
> new file mode 100644
> index 000000000000..401887118c09
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/mailbox/arm-smc.txt
> @@ -0,0 +1,101 @@
> +ARM SMC Mailbox Interface
> +=========================
> +
> +This mailbox uses the ARM smc (secure monitor 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[1].
> +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.
> +
> +Mailbox Device Node:
> +====================
> +
> +This node is expected to be a child of the /firmware node.
> +
> +Required properties:
> +--------------------
> +- compatible:		Shall be "arm,smc-mbox"
> +- #mbox-cells		Shall be 1 - the index of the channel needed.
> +- arm,num-chans		The number of channels supported.
> +- method:		A string, either:
> +			"hvc": if the driver shall use an HVC call, or
> +			"smc": if the driver shall use an SMC call.
> +
> +Optional properties:
> +- arm,func-ids		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.
> +- interrupts		SPI interrupts may be listed for notification,
> +			each channel should use a dedicated interrupt
> +			line.
> +

I think SMC mailbox as mostly unidirectional/Tx only channel. And the
interrupts here as stated are for notifications, so I prefer to keep
them separate channel. I assume SMC call return indicates completion.
Or do you plan to use these interrupts as the indication for completion
of the command? I see in patch 2/2 the absence of IRQ is anyway dealt
the way I mention above.

Does it make sense or am I missing something here ?

--
Regards,
Sudeep