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From:   Chen-Yu Tsai <wenst@chromium.org>
Date:   Fri, 3 Mar 2023 12:09:57 +0800
Message-ID: <CAGXv+5FwNfZ7TwKVMM5_uAjYQ6ZmhZVFsWREb_da-jxC6EUVJw@mail.gmail.com>
Subject: Re: [PATCH v4 12/19] arm64: dts: mediatek: mt8192-asurada: Couple
 VGPU and VSRAM_OTHER regulators
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
Cc:     matthias.bgg@gmail.com, robh+dt@kernel.org,
        krzysztof.kozlowski+dt@linaro.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
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On Thu, Mar 2, 2023 at 6:17=E2=80=AFPM AngeloGioacchino Del Regno
<angelogioacchino.delregno@collabora.com> wrote:
>
> Il 02/03/23 11:03, Chen-Yu Tsai ha scritto:
> > On Wed, Mar 1, 2023 at 5:55=E2=80=AFPM AngeloGioacchino Del Regno
> > <angelogioacchino.delregno@collabora.com> wrote:
> >>
> >> Add coupling for these regulators, as VSRAM_OTHER is used to power the
> >> GPU SRAM, and they have a strict voltage output relation to satisfy in
> >> order to ensure GPU stable operation.
> >> While at it, also add voltage constraint overrides for the GPU SRAM
> >> regulator "mt6359_vsram_others" so that we stay in a safe range of
> >> 0.75-0.80V.
> >>
> >> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@c=
ollabora.com>
> >> ---
> >>   arch/arm64/boot/dts/mediatek/mt8192-asurada.dtsi | 9 +++++++++
> >>   1 file changed, 9 insertions(+)
> >>
> >> diff --git a/arch/arm64/boot/dts/mediatek/mt8192-asurada.dtsi b/arch/a=
rm64/boot/dts/mediatek/mt8192-asurada.dtsi
> >> index 8570b78c04a4..f858eca219d7 100644
> >> --- a/arch/arm64/boot/dts/mediatek/mt8192-asurada.dtsi
> >> +++ b/arch/arm64/boot/dts/mediatek/mt8192-asurada.dtsi
> >> @@ -447,6 +447,13 @@ &mt6359_vrf12_ldo_reg {
> >>          regulator-always-on;
> >>   };
> >>
> >> +&mt6359_vsram_others_ldo_reg {
> >> +       regulator-min-microvolt =3D <750000>;
> >> +       regulator-max-microvolt =3D <800000>;
> >> +       regulator-coupled-with =3D <&mt6315_7_vbuck1>;
> >> +       regulator-coupled-max-spread =3D <10000>;
> >
> > Looking again at the downstream OPP table, it seems there's no voltage
> > difference requirement. It only needs V_SRAM >=3D V_GPU. Same applies t=
o
> > MT8195. Looks like only MT8183 and MT8186 need V_SRAM - V_GPU >=3D 1000=
0.
>
> On MT8195 we don't need any regulator coupling. There, the GPU-SRAM volta=
ge
> is fixed at .. I don't remember, 0.7V? - anyway - MT8195 doesn't need to
> scale the vsram.

Looks like it's fixed at 0.75V. I guess we're Ok on MT8195.

> >
> > Would setting max-spread to 0 work? I ask because with both regulator's
> > maximum voltage set to 0.8V, there's no way we can reach the highest
> > OPP.
> >
>
> No that doesn't work. I can raise the Vgpu max voltage to 0.88V to solve =
the
> issue right here and right now, or we can leave it like that and revisit =
it
> later.
>
> I would at this point go for setting mt6315_7_vbuck1's max-microvolt to
> 880000, as this is the maximum recommended voltage for the GPU as per the
> MT8192 datasheet, it would also make sense as we would be still describin=
g
> the hardware in a correct manner.
>
> What do you think?

If it's just to accommodate the coupler stuff, I say just set the maximum
at the lowest possible setting that satisfies the coupler constraint and
granularity of the regulator. The regulator does 6250 uV steps, so I guess
we could set the maximum at 812500 uV, with a comment stating the nominal
voltage of 800000 uV and that the extra 12500 uV is to workaround coupler
limitations.

Does that sound OK?

ChenYu