RE: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc for the ARM SMC/HVC mailbox

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

Hi Jassi,

> Subject: Re: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc for the ARM
> SMC/HVC mailbox
> 
> On Tue, Aug 27, 2019 at 10:02 PM Peng Fan <peng.fan@nxp.com> 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>
> > ---
> >  .../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
> > +---
> > +$id:
> > +https://eur01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdevi
> >
> +cetree.org%2Fschemas%2Fmailbox%2Farm-smc.yaml%23&amp;data=02%7
> C01%7Cp
> >
> +eng.fan%40nxp.com%7C8aa671dfa4d04ba003b508d72d0f297f%7C686ea1d
> 3bc2b4c
> >
> +6fa92cd99c5c301635%7C0%7C1%7C637027415448196145&amp;sdata=xd
> nUObNqlRF
> > +lu8NiXSuc35fYrHIzR%2Fyak6IzW05Q3nA%3D&amp;reserved=0
> > +$schema:
> > +https://eur01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdevi
> >
> +cetree.org%2Fmeta-schemas%2Fcore.yaml%23&amp;data=02%7C01%7Cpe
> ng.fan%
> >
> +40nxp.com%7C8aa671dfa4d04ba003b508d72d0f297f%7C686ea1d3bc2b4c6
> fa92cd9
> >
> +9c5c301635%7C0%7C1%7C637027415448196145&amp;sdata=wl%2Fdg09
> QMS%2FoHgI
> > +yD7ZBNpoIGXYxfFDRWhyYHogFd6A%3D&amp;reserved=0
> > +
> > +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.
> > +    items:
> > +      minimum: 1
> > +      maximum: 4096 # Should be enough?
> > +
> > +  method:
> > +    - enum:
> > +        - smc
> > +        - hvc
> > +
> > +  transports:
> > +    - 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.
> > +    $ref: /schemas/types.yaml#/definitions/uint32-array
> > +    minItems: 0
> > +    maxItems: 4096   # Should be enough?
> > +
> > +required:
> > +  - compatible
> > +  - "#mbox-cells"
> > +  - arm,num-chans
> > +  - transports
> > +  - method
> > +
> > +examples:
> > +  - |
> > +    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>;
> > +      };
> > +    };
> > +
> > +    firmware {
> > +      smc_mbox: mailbox {
> > +        #mbox-cells = <1>;
> > +        compatible = "arm,smc-mbox";
> > +        method = "smc";
> > +        arm,num-chans = <0x2>;
> > +        transports = "mem";
> > +        /* Optional */
> > +        arm,func-ids = <0xc20000fe>, <0xc20000ff>;
> >
> SMC/HVC is synchronously(block) running in "secure mode", i.e, there can
> only be one instance running platform wide. Right?

I think there could be channel for TEE, and channel for Linux.
For virtualization case, there could be dedicated channel for each VM.

  That implies there is only
> one physical channel in the platform.

I don't think so, TEE/Linux should use different physical channels,
i.e, SRAM memory partitioned using TZASC.

 So if you need to initiate different
> functions (tx, rx), you call them sequentially by changing the func-id for each
> request. Why not?

I could not follow you clearly. Could you please share more details?

Thanks,
Peng.
> 
> -Jassi
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