linux-kernel - Re: [PATCH 02/14] dt-bindings: soc: milbeaut: Add Milbeaut trampoline description

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Date:   Mon, 3 Dec 2018 09:49:29 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     sugaya.taichi@...ionext.com
Cc:     Stephen Boyd <sboyd@...nel.org>, devicetree@...r.kernel.org,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Russell King <linux@...linux.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Masami Hiramatsu <masami.hiramatsu@...aro.org>,
        Jassi Brar <jaswinder.singh@...aro.org>
Subject: Re: [PATCH 02/14] dt-bindings: soc: milbeaut: Add Milbeaut trampoline description

On Mon, Dec 3, 2018 at 1:42 AM Sugaya, Taichi
<sugaya.taichi@...ionext.com> wrote:
>
> Hi,
>
> On 2018/11/30 17:16, Stephen Boyd wrote:
> > Quoting Sugaya, Taichi (2018-11-29 04:24:51)
> >> On 2018/11/28 11:01, Stephen Boyd wrote:
> >>> Quoting Sugaya Taichi (2018-11-18 17:01:07)
> >>>>    create mode 100644 Documentation/devicetree/bindings/soc/socionext/socionext,m10v.txt
> >>>>
> >>>> diff --git a/Documentation/devicetree/bindings/soc/socionext/socionext,m10v.txt b/Documentation/devicetree/bindings/soc/socionext/socionext,m10v.txt
> >>>> new file mode 100644
> >>>> index 0000000..f5d906c
> >>>> --- /dev/null
> >>>> +++ b/Documentation/devicetree/bindings/soc/socionext/socionext,m10v.txt
> >>>> @@ -0,0 +1,12 @@
> >>>> +Socionext M10V SMP trampoline driver binding
> >>>> +
> >>>> +This is a driver to wait for sub-cores while boot process.
> >>>> +
> >>>> +- compatible: should be "socionext,smp-trampoline"
> >>>> +- reg: should be <0x4C000100 0x100>
> >>>> +
> >>>> +EXAMPLE
> >>>> +       trampoline: trampoline@...C000100 {
> >>> Drop the 0x part of unit addresses.
> >>
> >> Okay.
> >>
> >>
> >>>> +               compatible = "socionext,smp-trampoline";
> >>>> +               reg = <0x4C000100 0x100>;
> >>> Looks like a software construct, which we wouldn't want to put into DT
> >>> this way. DT doesn't describe drivers.
> >> We would like to use this node only getting the address of the
> >> trampoline area
> >> in which sub-cores wait.  (They have finished to go to this area in previous
> >> bootloader process.)
> >
> > Is this area part of memory, or a special SRAM? If it's part of memory,
> > I would expect this node to be under the reserved-memory node and
> > pointed to by some other node that uses this region. Could even be the
> > CPU nodes.
>
> Yes, 0x4C000100 is a part of memory under the reserved-memory node. So
> we would like to use the SRAM ( allocated 0x00000000 ) area instead.
> BTW, sorry, the trampoline address of this example is simply wrong.  We
> were going to use a part of the SRAM from the beginning.
>
> >
> >>
> >> So should we embed the constant value in source codes instead of getting
> >> from
> >> DT because the address is constant at the moment? Or is there other
> >> approach?
> >>
> >
> > If it's constant then that also works. Why does it need to come from DT
> > at all then?
>
> We think it is not good to embed constant value in driver codes and do
> not have another way...
> Are there better ways?

If this is just memory, can you use the standard spin-table binding in
the DT spec? There are some requirements like 64-bit values even on
32-bit machines (though this gets violated).

Rob