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

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

> Subject: Re: [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 <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
> > +---
> > +$id:
> > +https://eur01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdevi
> >
> +cetree.org%2Fschemas%2Fmailbox%2Farm-smc.yaml%23&amp;data=02%7
> C01%7Cp
> >
> +eng.fan%40nxp.com%7C37aa729c94944730868b08d72bbfc121%7C686ea1
> d3bc2b4c
> >
> +6fa92cd99c5c301635%7C0%7C1%7C637025974936865698&amp;sdata=Inp
> %2FLs39m
> > +Gv1fe3dZMSaGmgmyWPT6awPh47s3mEtQ%2BQ%3D&amp;reserved=0
> > +$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%7C37aa729c94944730868b08d72bbfc121%7C686ea1d3bc2b4c
> 6fa92cd9
> >
> +9c5c301635%7C0%7C1%7C637025974936865698&amp;sdata=jmoR1Qqm7
> 6N5NwDbgFE
> > +Fm8cpdW%2B%2FgqmG9mSGz9mXv58%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.
> 
> What do you mean by that ? I would prefer to drop the above line unless I am

Ok. Dropped it in v6.

> 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.

Since asynchronous notification dropped, so it should only be
synchronous communication could be established. So I'll
modify it as below:

r0/w0/x0 the firmware returns after the smc call is delivered as a received
message to the mailbox framework, so synchronous communication can be
established

> 
> > +  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 <sudeep.holla@arm.com>

Thanks,
Peng.

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

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