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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 13 Jul 2021 11:04:09 +0200
Message-ID: <CAMuHMdVG6eji_uW+7egeQH=77fwQnN_qQ4hRHgQa4XQYQrbL9Q@mail.gmail.com>
Subject: Re: [PATCH] arm64: dts: renesas: r9a07g044: Add missing GICv3 node properties
To:     Sudeep Holla <sudeep.holla@arm.com>
Cc:     "Lad, Prabhakar" <prabhakar.csengg@gmail.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>,
        Rob Herring <robh+dt@kernel.org>,
        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>
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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:
> > On Tue, Jul 13, 2021 at 10:22 AM Lad, Prabhakar
> > <prabhakar.csengg@gmail.com> wrote:
> > > On Tue, Jul 13, 2021 at 9:08 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > On Mon, Jun 14, 2021 at 2:48 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > > On Fri, Jun 11, 2021 at 5:21 PM Lad Prabhakar
> > > > > <prabhakar.mahadev-lad.rj@bp.renesas.com> wrote:
> > > > > > Add the below missing properties into GIC node,
> > > > > > - clocks
> > > > > > - clock-names
> > > > > > - power-domains
> > > > > > - resets
> > > > > >
> > > > > > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> > > > > > Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
> > > > >
> > > > > Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
> > > > >
> > > > > Queueing pending on[1].
> > > > >
> > > > > > [1] https://lore.kernel.org/linux-devicetree/
> > > > > >     20210609155108.16590-1-prabhakar.mahadev-lad.rj@bp.renesas.com/
> > > >
> > > > The dependency has been accepted, but this patch needs a respin
> > > > for the changed clocks.
> > > >
> > > Thank you for pointing this out. wrt resets the GIC has two signals
> > > (which I learnt lately when the dependency path was accepted). Earlier
> > > discussion in irc with Sudeep pointed out that there wouldn't be any
> > > use case of having GIC resets in DTSI, so either we drop the resets
> > > property in DT binding doc or correct it.
> > >
> > > Let me know your thoughts on this and how we proceed further.
> >
> > DT Rule #1: DT describes hardware not software policy.
> >
>
> Completely agreed, no disagreement .

Good ;-)

> > 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, ...

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.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds