linux-kernel - Re: [PATCH v12 4/4] arm64: dts: qcom: sc7280-herobrine: Add lpi pinmux properties for CRD 3.0/3.1

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Date:   Thu, 5 May 2022 17:06:08 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Srinivasa Rao Mandadapu <quic_srivasam@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        quic_rohkumar@...cinc.com,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Judy Hsiao <judyhsiao@...omium.org>,
        Venkata Prasad Potturu <quic_potturu@...cinc.com>
Subject: Re: [PATCH v12 4/4] arm64: dts: qcom: sc7280-herobrine: Add lpi
 pinmux properties for CRD 3.0/3.1

Hi,

On Wed, May 4, 2022 at 10:07 AM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
> On Fri 29 Apr 11:10 CDT 2022, Doug Anderson wrote:
>
> > Hi,
> >
> > On Thu, Apr 28, 2022 at 5:02 PM Matthias Kaehlcke <mka@...omium.org> wrote:
> > >
> > > On Wed, Apr 27, 2022 at 10:39:43PM +0530, Srinivasa Rao Mandadapu wrote:
> > > > Add LPASS LPI pinctrl properties, which are required for Audio
> > > > functionality on herobrine based platforms of rev5+
> > > > (aka CRD 3.0/3.1) boards.
> > > >
> > > > Signed-off-by: Srinivasa Rao Mandadapu <quic_srivasam@...cinc.com>
> > > > Co-developed-by: Venkata Prasad Potturu <quic_potturu@...cinc.com>
> > > > Signed-off-by: Venkata Prasad Potturu <quic_potturu@...cinc.com>
> > >
> > > I'm not super firm in pinctrl territory, a few maybe silly questions
> > > below.
> > >
> > > >  arch/arm64/boot/dts/qcom/sc7280-herobrine-crd.dts | 84 +++++++++++++++++++++++
> > > >  1 file changed, 84 insertions(+)
> > > >
> > > > diff --git a/arch/arm64/boot/dts/qcom/sc7280-herobrine-crd.dts b/arch/arm64/boot/dts/qcom/sc7280-herobrine-crd.dts
> > > > index deaea3a..dfc42df 100644
> > > > --- a/arch/arm64/boot/dts/qcom/sc7280-herobrine-crd.dts
> > > > +++ b/arch/arm64/boot/dts/qcom/sc7280-herobrine-crd.dts
> > > > @@ -111,6 +111,90 @@ ap_ts_pen_1v8: &i2c13 {
> > > >   * - If a pin is not hooked up on Qcard, it gets no name.
> > > >   */
> > > >
> > > > +&lpass_dmic01 {
> > > > +     clk {
> > > > +             drive-strength = <8>;
> > > > +     };
> >
> > Ugh, I've been distracted and I hadn't realized we were back to the
> > two-level syntax. Definitely not my favorite for all the reasons I
> > talked about [1]. I guess you took Bjorn's silence to my response to
> > mean that you should switch back to this way? :(
> >
> > Bjorn: can you clarify?
> >
>
> I didn't think through the fact that &mi2s0_state was specified in the
> .dtsi and as such will be partially be overridden by the baord dts.
>
>
> I do prefer the two level style and describing full "states", but as you
> say whenever we provide something that will have to be overwritten it's
> suboptimal.
>
> As such, I think your flattened model is preferred in this case

How about for future patches we just provided labels at both levels
(I'm not suggesting we churn this patch series more):

lpass_dmic01_sleep: dmic01-sleep {
  lpass_dmic01_sleep_clk: clk {
    pins = "gpio6";
    function = "dmic1_clk";
  };

  lpass_dmic01_sleep_data: data {
    pins = "gpio7";
    function = "dmic1_data";
  };
};

Then you can in your pinctrl reference you can just reference the
top-level node but boards can override without having to replicate
hierarchy...

> but it
> makes me dislike the partial definition between the dtsi and dts even
> more (but I don't have any better suggestion).

One other proposal I'd make is that maybe we should change the rules
about never putting drive strength in the soc.dtsi file. While it
should still be OK for boards to override the drive strength, it seems
like a whole lot of biolerplate code to have every board override
every pin and say that its drive strength is 2. Similarly, if there's
a high speed interface (like eMMC) where a drive strength of 2 is
nonsense for any board, it doesn't seem ridiculous to specify a
default drive strength of something higher in the soc.dtsi file.

I would like to say the same thing goes for for pulls, but it's
unfortunately uglier for pulls. :( For instance, nearly everyone has
an external pullup for i2c busses. The strength of the pullup needs to
be tuned for the i2c bus speed and the impedance of the line. Thus, it
would ideally make sense to specify this in the soc.dtsi file.
Unfortunately, if we do that and some board _wants_ to use the
internal pulls (maybe they're running at a really low speed and/or
forgot to add external pulls) then they have to do an ugly
"/delete-property/ bias-disable" because adding the "bias-pull-up"
doesn't delete the other property and you end up with both. :( That
seems bad, so I guess I'd vote to keep banning bias definitions in the
soc.dtsi file.

Anyway, I'd love your opinion on this.

-Doug