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

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Message-ID: <AM0PR04MB4481632406DF235996719C0788170@AM0PR04MB4481.eurprd04.prod.outlook.com>
Date:   Thu, 6 Jun 2019 03:24:38 +0000
From:   Peng Fan <peng.fan@....com>
To:     Andre Przywara <andre.przywara@....com>,
        Sudeep Holla <sudeep.holla@....com>
CC:     Florian Fainelli <f.fainelli@...il.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "jassisinghbrar@...il.com" <jassisinghbrar@...il.com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "festevam@...il.com" <festevam@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "van.freenix@...il.com" <van.freenix@...il.com>
Subject: RE: [PATCH V2 1/2] DT: mailbox: add binding doc for the ARM SMC
 mailbox

> Subject: Re: [PATCH V2 1/2] DT: mailbox: add binding doc for the ARM SMC
> mailbox
> 
> On Mon, 3 Jun 2019 17:56:51 +0100
> Sudeep Holla <sudeep.holla@....com> wrote:
> 
> Hi,
> 
> > On Mon, Jun 03, 2019 at 09:22:16AM -0700, Florian Fainelli wrote:
> > > On 6/3/19 1:30 AM, peng.fan@....com wrote:
> > > > From: Peng Fan <peng.fan@....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@....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 @@
> >
> > [...]
> >
> > > > +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 would not go about defining a specific kind of interrupt, since
> > > SPI is a GIC terminology, this firmware interface could be used in
> > > premise with any parent interrupt controller, for which the concept
> > > of a SPI/PPI/SGI may not be relevant.
> > >
> >
> > While I agree the binding document may not contain specifics, I still
> > don't see how to use SGI with this. Also note it's generally reserved
> > for OS future use(IPC) and using this for other than IPC may be bit
> > challenging IMO. It opens up lots of questions.
> 
> Well, a PPI might be possible to use, although it's somewhat dodgy to hijack
> the GIC's (re-)distributor from EL3 to write to GICD_ISPENDR<n>. Need to ask
> Marc about his feelings towards this. But it's definitely possible from a
> hypervisor to inject arbitrary interrupts into a guest.
> 
> But more importantly: is there any actual reason this needs to be a GIC
> interrupt? 

No. I just test ATF with SPI when I posting out this. Should not restrict to be GIC.

If I understand the code correctly, this could just be any interrupt,
> including one of an interrupt combiner or a GPIO chip. So why not just use the
> standard wording of: "exactly one interrupt specifier for each channel"?

Agree.

> 
> By the way: Shouldn't new bindings use the YAML format instead?

I'll convert to YAML in next version.

Thanks,
Peng.

> 
> Cheers,
> Andre.