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Date:   Tue, 13 Jul 2021 10:15:02 +0100
From:   Sudeep Holla <sudeep.holla@arm.com>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     "Lad, Prabhakar" <prabhakar.csengg@gmail.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>,
        Rob Herring <robh+dt@kernel.org>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Magnus Damm <magnus.damm@gmail.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        Biju Das <biju.das.jz@bp.renesas.com>
Subject: Re: [PATCH] arm64: dts: renesas: r9a07g044: Add missing GICv3 node
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On Tue, Jul 13, 2021 at 11:04:09AM +0200, Geert Uytterhoeven wrote:
> Hi Sudeep,
> 
> On Tue, Jul 13, 2021 at 10:56 AM Sudeep Holla <sudeep.holla@arm.com> wrote:
> > On Tue, Jul 13, 2021 at 10:30:36AM +0200, Geert Uytterhoeven wrote:

[...]

> > > And a possible use case: the RT CPU core may want to reset the AP GIC.
> >
> > I didn't want to add new bindings without details on the implementation
> > to avoid possible issues with backward compatibility as this was not
> > thought through completely and correctly before it was added.
> >
> > OK, now let us discuss your use-case: *RT CPU wants to reset AP GIC*
> >
> > 1. Will it just reset AP GIC or will it request the AP reset as a whole ?
> >    I am not sure if we can handle former, if you think otherwise what is
> >    the reset notification mechanism ?
> >
> > 2. Will that bypass secure world/PSCI ? Again more details on this would
> >    be helpful to visualise the entire use-case end-to-end better.
> >
> > By GIC reset, I am assuming it will be complete GIC reset including it's
> > CPU interface.
> >
> > I don't think we can reset GIC without actual CPU reset. Even if we get
> > some notification magically to the CPU that its GIC alone needs to be
> > reset, it needs to safely higher exceptions to get its GIC CPU interface
> > reprogrammed to correct (saved) values before OS can reprogram the NS
> > world values. All these seems overall complicated and may be unnecessary.
>
> Probably both.  Might make sense to reset on wake-up, after having disabled
> clocks and powered down the AP CPU, AP GIC, ...
>

/me confused. If this is arm64 platform, then you have to use *PSCI* and
I expect the reset to be done as part of CPU wake-up in PSCI firmware.

> If that bypasses PSCI: well, if the unsecure software can do it, it
> means the hardware is not secure. Or at least Linux has to be trusted.
>

No, if the system has PSCI, then you simply can't bypass that for GIC
reset. Or at-least I am failing to understand the complete flow of that.

--
Regards,
Sudeep