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Date:   Tue, 6 Feb 2018 12:05:27 -0800
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Doug Anderson <dianders@chromium.org>
Cc:     Stephen Boyd <sboyd@codeaurora.org>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        Andy Gross <andy.gross@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-msm@vger.kernel.org,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        devicetree@vger.kernel.org
Subject: Re: [PATCH 2/2] arm64: dts: sdm845: Add serial console support
Message-ID: <20180206200527.GM9465@builder>
References: <20180125163216.29018-1-rnayak@codeaurora.org>
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 <20180126221808.GE28313@codeaurora.org>
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On Tue 06 Feb 11:49 PST 2018, Doug Anderson wrote:

> Hi,
> 
> On Tue, Feb 6, 2018 at 11:06 AM, Bjorn Andersson
> <bjorn.andersson@linaro.org> wrote:
> > On Tue 06 Feb 10:37 PST 2018, Doug Anderson wrote:
> >
> >> Hi,
> >>
> >> On Fri, Jan 26, 2018 at 2:18 PM, Stephen Boyd <sboyd@codeaurora.org> wrote:
> >> > On 01/25, Rajendra Nayak wrote:
> >> >> diff --git a/arch/arm64/boot/dts/qcom/sdm845-pins.dtsi b/arch/arm64/boot/dts/qcom/sdm845-pins.dtsi
> >> >> new file mode 100644
> >> >> index 000000000000..b97f99e6f4b4
> >> >> --- /dev/null
> >> >> +++ b/arch/arm64/boot/dts/qcom/sdm845-pins.dtsi
> >> >> @@ -0,0 +1,32 @@
> >> >> +// SPDX-License-Identifier: GPL-2.0
> >> >> +/*
> >> >> + * Copyright (c) 2018, The Linux Foundation. All rights reserved.
> >> >> + */
> >> >> +
> >> >> +&tlmm {
> >> >
> >> > I'm not the maintainer, but I find this approach to the pins
> >> > really annoying. I have to flip to another file to figure out how
> >> > a board has configured the pins. And we may bring in a bunch of
> >> > settings that we don't ever use on some board too. Why can't we
> >> > put the settings in the board file directly?
> >>
> >> I'm not so familiar with how things work with Qualcomm, but in general
> >> I think putting this in the "board" file is a bad idea.  I'd be OK
> >> with putting this directly in the SoC file (though it might get
> >> unwieldy?), but not moving things to the board file as was done with
> >> v2 of this patch.
> >>
> >> Said another way: nearly board that uses SDM845 that uses UART2 will
> >> have the same definitions for these pins so we shouldn't be
> >> duplicating it across every board, right?
> >>
> >
> > We've run into several cases where different boards uses the same
> > function but requires board specific electrical configuration.
> >
> > So what we decided was to keep the pinmux in the soc-file (where e.g.
> > the uart definition is) and then extend it with the board specific
> > electrical properties (the pinconf), in the board files.
> >
> > This does come with the complexity of having the pinctrl nodes split in
> > two places, but the responsibilities of the two parts is clear and we
> > remove the need for all board files to ensure the appropriate pinmux is
> > in place.
> >
> >
> > NB. We did discuss adding "sane defaults" for the pinconf in the soc
> > dtsi, but we end up spending considerable time debugging issues stemming
> > from not having the right pinconf; so better make this explicit and say
> > that the board has to specify it's config.
> 
> Whoops, saw your responses _after_ I sent my response to v2.  In any
> case this makes sense to me then!  On Rockchip boards I've been
> involved in we often added "sane defaults", but I can see how that
> could be confusing in different ways.  I'm happy with your choice and
> it seems like a happy medium.  The sdm845.dtsi file can have the main
> definition of the nodes and can thus refer to the nodes.  Then you
> just add the extra bit in the board file.
> 
> What you propose is not what happened in v2 of the series
> <https://patchwork.kernel.org/patch/10194201/> though.  In v2 _both_
> the pinconf and the pinmux moved to the board file.  That's wrong.
> 
> 
> To make it concrete, you'd have something like this (this has the
> wrong bindings from the UART, but folks get the picture hopefully):
> 
> 
> In sdm845.dtsi:
> 
>     qup_uart2: serial@a84000 {
>       compatible = "qcom,geni-console", "qcom,geni-uart";
>       reg = <0xa84000 0x4000>;
>       reg-names = "se_phys";
>       clock-names = "se-clk", "m-ahb", "s-ahb";
>       clocks = <&gcc GCC_QUPV3_WRAP1_S1_CLK>,
>          <&gcc GCC_QUPV3_WRAP_1_M_AHB_CLK>,
>          <&gcc GCC_QUPV3_WRAP_1_S_AHB_CLK>;
>       pinctrl-names = "default", "sleep";
>       pinctrl-0 = <&qup_uart2_default>;
>       pinctrl-1 = <&qup_uart2_sleep>;
>       interrupts = <GIC_SPI 354 IRQ_TYPE_NONE>;
>       qcom,wrapper-core = <&qup_1>;
>       status = "disabled";
>     };
> 
>     tlmm: pinctrl@3400000 {
>       compatible = "qcom,sdm845-pinctrl";
>       reg = <0x03400000 0xc00000>;
>       interrupts = <GIC_SPI 208 IRQ_TYPE_NONE>;
>       gpio-controller;
>       #gpio-cells = <2>;
>       interrupt-controller;
>       #interrupt-cells = <2>;
> 
>       qup_uart2_default: qup_uart2_default {
>         pinmux {
>           function = "qup9";
>           pins = "gpio4", "gpio5";
>         };
>       };
> 
>       qup_uart2_sleep: qup_uart2_sleep {
>         pinmux {
>           function = "gpio";
>           pins = "gpio4", "gpio5";
>         };
>       };
>     };
> 
> In sdm845-mtp.dts:
> 
> &qup_uart2_default {
>   pinconf {
>     pins = "gpio4", "gpio5";
>     drive-strength = <2>;
>     bias-disable;
>   };
> };
> 
> &qup_uart2_sleep {
>   pinconf {
>     pins = "gpio4", "gpio5";
>     drive-strength = <2>;
>     bias-disable;
>   };
> };

Correct.


This example does however show another thing that I really do not like;
When you have a lot of nodes I find it very useful to maintain some sort
of grouping, to know that I can find a node describing properties
related to some block close to related blocks - e.g. nodes describing
a pmic block is close to other nodes for that pmic.

Today we seem to have a mixture of bus-based grouping, arbitrary
grouping and no grouping at all in our upstream dtsi files, so I think
we should set some guidelines here as well.

Regards,
Bjorn