linux-kernel - Re: [PATCH v1 5/6] arm64: dts: meson: a1: introduce UART

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Date:   Wed, 28 Jun 2023 17:49:34 +0300
From:   Dmitry Rokosov <ddrokosov@...rdevices.ru>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
CC:     <neil.armstrong@...aro.org>, <jbrunet@...libre.com>,
        <mturquette@...libre.com>, <sboyd@...nel.org>,
        <robh+dt@...nel.org>, <krzysztof.kozlowski+dt@...aro.org>,
        <khilman@...libre.com>, <conor+dt@...nel.org>,
        <kernel@...rdevices.ru>, <sdfw_system_team@...rdevices.ru>,
        <rockosov@...il.com>, <linux-amlogic@...ts.infradead.org>,
        <linux-clk@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Oleg Lyovin <ovlevin@...rdevices.ru>
Subject: Re: [PATCH v1 5/6] arm64: dts: meson: a1: introduce UART_AO mux
 definitions

Martin,

On Sun, Jun 25, 2023 at 11:07:51PM +0200, Martin Blumenstingl wrote:
> On Wed, Jun 7, 2023 at 10:16 PM Dmitry Rokosov <ddrokosov@...rdevices.ru> wrote:
> >
> > From: Oleg Lyovin <ovlevin@...rdevices.ru>
> >
> > The Amlogic A1 has a UART_AO port, which can be used, for example, for
> > BT HCI H4 connection.
> >
> > This patch adds mux definitions for it.
> In the past we've only taken the pinctrl definitions if we have a
> board that uses them.
> Neil, do we still have the same policy in place? If so this patch
> should be sent with the series that adds support for your A1 board.
> 
> > Signed-off-by: Oleg Lyovin <ovlevin@...rdevices.ru>
> > Signed-off-by: Dmitry Rokosov <ddrokosov@...rdevices.ru>
> > ---
> >  arch/arm64/boot/dts/amlogic/meson-a1.dtsi | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/arch/arm64/boot/dts/amlogic/meson-a1.dtsi b/arch/arm64/boot/dts/amlogic/meson-a1.dtsi
> > index 0efd922ca7e1..3eb6aa9c00e0 100644
> > --- a/arch/arm64/boot/dts/amlogic/meson-a1.dtsi
> > +++ b/arch/arm64/boot/dts/amlogic/meson-a1.dtsi
> > @@ -118,6 +118,22 @@ gpio: bank@400 {
> >                                         gpio-ranges = <&periphs_pinctrl 0 0 62>;
> >                                 };
> >
> > +                               uart_a_pins: uart_a {
> Only our newer .dtsi (e.g. meson-g12-common.dtsi) are following the
> pattern where node names should use dashes instead of underscores.
> So please use: uart_a_pins: uart-a { ...
> 

Okay, no problem. I'll rename the nodes in the next patch series
version.

> [...]
> > +                               uart_a_cts_rts_pins: uart_a_cts_rts {
> similar to the comment from above:
> uart_a_cts_rts_pins: uart-a-cts-rts { ...
> 

Ok

> > +                                       mux {
> > +                                               groups = "uart_a_cts",
> > +                                                        "uart_a_rts";
> > +                                               function = "uart_a";
> > +                                               bias-pull-down;
> Out of curiosity: is this pull down needed on all boards or just specific ones?
> It seems like all other SoCs use bias-disable for the RTS/CTS pins.
> 

That's a good question. The Amlogic custom kernel DTSI declares
bias-pull-down for CTS/RTS pins in UART_A. There is no information about
this in the A1 datasheet. However, from my understanding, it is related
to the usage of the UART_A. Typically, the UART_A endpoint on A1 boards
is used for BT connections, which is why Amlogic applies bias-pull-down
in the common DTSI. If my assumption is correct, it would be better to
move the bias-pull-down node parameter to the custom board DTS. I will
investigate this further and rework it in the next version if necessary.

-- 
Thank you,
Dmitry