RE: [PATCH 1/2] dt-bindings: arm: arm,scmi: add smc/hvc transports

From: Peng Fan <peng.fan@nxp.com>
To: Sudeep Holla <sudeep.holla@arm.com>, Marc Zyngier <maz@kernel.org>
Cc: "robh+dt@kernel.org" <robh+dt@kernel.org>,
	"mark.rutland@arm.com" <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"f.fainelli@gmail.com" <f.fainelli@gmail.com>,
	"viresh.kumar@linaro.org" <viresh.kumar@linaro.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	dl-linux-imx <linux-imx@nxp.com>,
	"andre.przywara@arm.com" <andre.przywara@arm.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: RE: [PATCH 1/2] dt-bindings: arm: arm,scmi: add smc/hvc transports
Date: Fri, 7 Feb 2020 10:55:44 +0000	[thread overview]
Message-ID: <AM0PR04MB4481B1D5E2725E85BC6D6D71881C0@AM0PR04MB4481.eurprd04.prod.outlook.com> (raw)
In-Reply-To: <20200207104736.GB36345@bogus>

> Subject: Re: [PATCH 1/2] dt-bindings: arm: arm,scmi: add smc/hvc transports
> 
> On Fri, Feb 07, 2020 at 10:08:36AM +0000, Marc Zyngier wrote:
> > On 2020-02-06 13:01, peng.fan@nxp.com wrote:
> > > From: Peng Fan <peng.fan@nxp.com>
> > >
> > > SCMI could use SMC/HVC as tranports, so add into devicetree binding
> > > doc.
> > >
> > > Signed-off-by: Peng Fan <peng.fan@nxp.com>
> > > ---
> > >  Documentation/devicetree/bindings/arm/arm,scmi.txt | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/Documentation/devicetree/bindings/arm/arm,scmi.txt
> > > b/Documentation/devicetree/bindings/arm/arm,scmi.txt
> > > index f493d69e6194..03cff8b55a93 100644
> > > --- a/Documentation/devicetree/bindings/arm/arm,scmi.txt
> > > +++ b/Documentation/devicetree/bindings/arm/arm,scmi.txt
> > > @@ -14,7 +14,7 @@ Required properties:
> > >
> > >  The scmi node with the following properties shall be under the
> > > /firmware/ node.
> > >
> > > -- compatible : shall be "arm,scmi"
> > > +- compatible : shall be "arm,scmi" or "arm,scmi-smc"
> > >  - mboxes: List of phandle and mailbox channel specifiers. It should
> > > contain
> > >  	  exactly one or two mailboxes, one for transmitting messages("tx")
> > >  	  and another optional for receiving the notifications("rx") if @@
> > > -25,6 +25,8 @@ The scmi node with the following properties shall be
> > > under the /firmware/ node.
> > >  	  protocol identifier for a given sub-node.
> > >  - #size-cells : should be '0' as 'reg' property doesn't have any size
> > >  	  associated with it.
> > > +- arm,smc-id : SMC id required when using smc transports
> > > +- arm,hvc-id : HVC id required when using hvc transports
> > >
> > >  Optional properties:
> >
> > Not directly related to DT: Why do we need to distinguish between SMC
> > and HVC?
> 
> IIUC you want just one property to get the function ID ? Does that align with
> what you are saying ? I wanted to ask the same question and I see no need for
> 2 different properties.

The multiple protocols might use SMC or HVC. Saying

 Protocol@x {
    method="smc";
    arm,func-id=<0x....>
 };
 Protocol@y {
    method="hvc";
    arm,func-id=<0x....>
 };

With my propose:

Protocol@x {
    arm,smc-id=<0x....>
 };
 Protocol@y {
    arm,hvc-id=<0x....>
 };

No need an extra method property to indicate it is smc or hvc.
The driver use take arm,smc-id as SMC, arm,hvc-id as HVC.

Thanks,
Peng.

> 
> > Other SMC/HVC capable protocols are able to pick the right one based
> > on the PSCI conduit.
> >
> 
> This make it clear, but I am asking to  be sure.
> 
> > This is how the Spectre mitigations work already. Why is that any different?
> >
> 
> I don't see any need for it to be different.
> 
> --
> Regards,
> Sudeep