Date:   Thu, 20 Jun 2019 11:27:19 -0500
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Andre Przywara <andre.przywara@....com>
Cc:     Sudeep Holla <sudeep.holla@....com>, Peng Fan <peng.fan@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Florian Fainelli <f.fainelli@...il.com>,
        ", Sascha Hauer" <kernel@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>,
        Shawn Guo <shawnguo@...nel.org>, festevam@...il.com,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org, van.freenix@...il.com
Subject: Re: [PATCH V2 1/2] DT: mailbox: add binding doc for the ARM SMC mailbox

On Thu, Jun 20, 2019 at 11:13 AM Andre Przywara <andre.przywara@....com> wrote:
>
> On Thu, 20 Jun 2019 10:22:41 +0100
> Sudeep Holla <sudeep.holla@....com> wrote:
>
> > On Mon, Jun 03, 2019 at 04:30:04PM +0800, 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 @@
> > > +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.
> > > +
> >
> > I think SMC mailbox as mostly unidirectional/Tx only channel. And the
> > interrupts here as stated are for notifications, so I prefer to keep
> > them separate channel. I assume SMC call return indicates completion.
> > Or do you plan to use these interrupts as the indication for completion
> > of the command? I see in patch 2/2 the absence of IRQ is anyway dealt
> > the way I mention above.
> >
> > Does it make sense or am I missing something here ?
>
> I think you are right. From a mailbox point of view "completion" means
> that the trigger has reached the other side. A returning smc call is a
> perfect indication of this fact.
>
Yes. mailbox only cares about message delivery.

> Whether the action triggered by this
> mailbox command has completed is a totally separate question and out of
> the scope of the mailbox.
>
Yes, whether the message is accepted/rejected at protocol level is a
matter of upper layer (protocol).

> This should be handled by a higher level
> protocol (SCPI in this case). Which could mean that this employs a
> separate return mailbox channel, which is RX only and implemented by
> interrupts. Which could or could not be part of this driver.
>
Any message received over the same class of channel should be handled
in this driver.

Cheers

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