linux-kernel - Re: [PATCH v4 1/2] dt-bindings: Add bindings for BrcmSTB SCMI mailbox driver

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Date:   Thu, 5 Nov 2020 10:28:25 -0500
From:   Jim Quinlan <james.quinlan@...adcom.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        "maintainer:BROADCOM BCM7XXX ARM ARCHITECTURE" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "moderated list:BROADCOM BCM7XXX ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] dt-bindings: Add bindings for BrcmSTB SCMI mailbox driver

On Thu, Nov 5, 2020 at 10:13 AM Rob Herring <robh@...nel.org> wrote:
>
> On Wed, Nov 4, 2020 at 4:04 PM Jim Quinlan <james.quinlan@...adcom.com> wrote:
> >
> > On Wed, Nov 4, 2020 at 4:50 PM Rob Herring <robh@...nel.org> wrote:
> > >
> > > On Thu, Oct 29, 2020 at 03:59:06PM -0400, Jim Quinlan wrote:
> > > > Bindings are added.  Only one interrupt is needed because
> > > > we do not yet employ the SCMI p2a channel.
> > >
> > > I still don't understand what this is. To repeat from v1: I thought SCMI
> > > was a mailbox consumer, not provider?
> >
> > Hi Rob,
> >
> > I'm not sure where I am implying that SCMI is a mailbox provider?
> > Should I not mention "SCMI" in the subject line?
> >
> > This is just a mailbox driver, "consumed" by SCMI.    Our SCMI DT node
> > looks like this:
> >
> > brcm_scmi_mailbox: brcm_scmi_mailbox@0 {
> >         #mbox-cells = <1>;
> >         compatible = "brcm,brcmstb-mbox";
> > };
> >
> > brcm_scmi@0 {
> >         compatible = "arm,scmi";
> >         mboxes = <&brcm_scmi_mailbox 0>;;
> >         mbox-names = "tx";
> >         shmem = <&NWMBOX>;
> >         /* ... */
> > };
>
> Okay, that makes more sense. Though it seems like this is just adding
> a pointless level of indirection to turn an interrupt into a mailbox.
> There's nothing more to 'the mailbox' is there?
Correct.  Although you can see that it uses both interrupts and SMC
calls to get the job done.

> So why not either
> allow SCMI to have an interrupt directly
Not sure here -- perhaps the SCMI folks have an answer?

 > or have a generic irq mailbox
> driver?
The SCMI implementation doesn't offer a generic irq mailbox driver
AFAICT.  The SCMI folks recently provided  an "smc transport" driver
in "drivers/firmware/arm_scmi/smc.c" -- it is close to what we need
but is missing interrupts.

Regards,
Jim Quinlan
Broadcom STB

>
> Rob

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