linux-kernel - Re: [PATCH] arm64: dts: mt8173-oak: Switch to SMC watchdog

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Message-ID: <CAEXTbpeEkpgFjQfONM030k8drcfYzOoWnB_FUqFptd-27YRqNg@mail.gmail.com>
Date:   Tue, 26 Jul 2022 13:55:02 +0800
From:   Pin-yen Lin <treapking@...omium.org>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
Cc:     Matthias Brugger <matthias.bgg@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Eizan Miyamoto <eizan@...omium.org>,
        Evan Benn <evanbenn@...omium.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: mt8173-oak: Switch to SMC watchdog

On Mon, Jul 25, 2022 at 6:31 PM AngeloGioacchino Del Regno
<angelogioacchino.delregno@...labora.com> wrote:
>
> Il 25/07/22 12:19, Pin-yen Lin ha scritto:
> > On Mon, Jul 25, 2022 at 4:39 PM AngeloGioacchino Del Regno
> > <angelogioacchino.delregno@...labora.com> wrote:
> >>
> >> Il 25/07/22 10:24, Pin-yen Lin ha scritto:
> >>> Switch to SMC watchdog because we need direct control of HW watchdog
> >>> registers from kernel. The corresponding firmware was uploaded in
> >>> https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/3405.
> >>>
> >>
> >> There's a fundamental issue with this change, I think.
> >>
> >> What happens if we run this devicetree on a device that does *not* have
> >> the new(er) firmware?
> >
> > I haven't tried this patch with an older firmware. I'll manage to
> > build one for this.
> >>
> >> The kernel *shall not* get broken when running on devices that are running
> >> on older firmware, especially because that's what was initially supported
> >> and what is working right now.
> >
> > Actually the current approach does not work *right*. The device boots,
> > but the watchdog does not work properly.
> >
>
> Is this a Chromebook firmware specific issue?

I'm not sure if this is a Chromebook-specific issue. The internal
issue thread only discussed this for the Chromebook firmware.
>
> > Also, all MT8173 ChromeOS devices have this firmware updated, and we
> > don't have other upstream users apart from mt8173-evb. Do we want to
> > support the developers that are running upstream linux with their
> > MT8173 boards?
> >
>
> Upstream shall not be just about one machine: if we add support for a SoC there,
> we shall support the SoC-generic things in the SoC-specific DTSI, and the machine
> specific things in the machine-specific devicetrees.
>
> Chromebooks are not the only machines using the MT8173 SoC (Chuwi, Amazon also do
> have products using MT8173), so we shall not make the main mt8173.dtsi incompatible
> with these machines.

I don't see their DTS files uploaded to the upstream kernel. So we
still want to support them even if they didn't upstream their changes?

Does it make sense if we move the modification to mt8173-elm.dtsi? The
device should be running ChromeOS AP firmware if it uses or references
mt8173-elm.dtsi. Also, all the MT8173 Chromebooks were shipped with
the "new" firmware from the very beginning. We just somehow didn't
upstream this around the time.
>
> >>
> >> For this reason, I think that we should get some code around that checks
> >> if the SMC watchdog is supported and, if not, resort to MMIO wdog.
> >
> > What is the expected way to support this backward compatibility? Do we
> > put the old compatible strings ("mediatek,mt8173-wdt" and
> > "mediatek,mt6589-wdt") after "arm,smc-wdt" and reject it in the
> > drivers if the firmware does not support it?
>
> I don't know what's the best option to support both cases... Perhaps a good one
> would be to check (in mtk_wdt? or in arm_smc_wdt?) if the arm_smc_wdt is actually
> supported in firmware, so if the SMC one is registered, we skip the other.
>
> >>
> >> Regards,
> >> Angelo
> >>
> >>
> >>> Signed-off-by: Pin-yen Lin <treapking@...omium.org>
> >>> ---
> >>>
> >>>    arch/arm64/boot/dts/mediatek/mt8173.dtsi | 6 ++----
> >>>    1 file changed, 2 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/arch/arm64/boot/dts/mediatek/mt8173.dtsi b/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> >>> index a2aef5aa67c1..2d1c776740a5 100644
> >>> --- a/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> >>> +++ b/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> >>> @@ -528,10 +528,8 @@ power-domain@...173_POWER_DOMAIN_MFG {
> >>>                        };
> >>>                };
> >>>
> >>> -             watchdog: watchdog@...07000 {
> >>> -                     compatible = "mediatek,mt8173-wdt",
> >>> -                                  "mediatek,mt6589-wdt";
> >>> -                     reg = <0 0x10007000 0 0x100>;
> >>> +             watchdog {
> >>> +                     compatible = "arm,smc-wdt";
> >>>                };
> >>>
> >>>                timer: timer@...08000 {
> >>>
> >>
> >>
>
>