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

[Peng Fan <peng.fan@nxp.com>]

Hi Rob,

> Subject: Re: [PATCH V2 1/2] DT: mailbox: add binding doc for the ARM SMC
> mailbox
> 
> 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.
> > +
> > +Example:
> > +--------
> > +
> > +	sram@910000 {
> > +		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>;
> > +		};
> > +	};
> > +
> > +	smc_mbox: mailbox {
> 
> This should be a child of 'firmware' node at least and really a child of the
> firmware component that implements the feature.

I checked other mbox driver, including the mbox used by ti sci, mbox used by
i.MX8QXP. both mbox dts node not a child a firmware node,
I am not sure why put mbox node into a child a firmware node here.

Thanks,
Peng.

> 
> > +		#mbox-cells = <1>;
> > +		compatible = "arm,smc-mbox";
> > +		method = "smc";
> > +		arm,num-chans = <0x2>;
> > +		/* Optional */
> > +		arm,func-ids = <0xc20000fe>, <0xc20000ff>;
> > +	};
> > +
> > +	firmware {
> > +		scmi {
> > +			compatible = "arm,scmi";
> > +			mboxes = <&mailbox 0 &mailbox 1>;
> > +			mbox-names = "tx", "rx";
> > +			shmem = <&cpu_scp_lpri &cpu_scp_hpri>;
> > +		};
> > +	};
> > +
> > +
> > +[1]
> > +https://eur01.safelinks.protection.outlook.com/?url=http%3A%2F%2Finfo
> >
> +center.arm.com%2Fhelp%2Findex.jsp%3Ftopic%3D%2Fcom.arm.doc.den002
> 8a%2
> >
> +Findex.html&amp;data=02%7C01%7Cpeng.fan%40nxp.com%7Cd8cf8b81b4f
> b49be5
> >
> +97c08d703f26576%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C1%7
> C63698221
> >
> +1931902513&amp;sdata=RtHkNN07b%2FuzdJkiu0QujeJ6czrcwOwEI6Y6JW
> VpPkY%3D
> > +&amp;reserved=0
> > --
> > 2.16.4
> >