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From:   Javier Martinez Canillas <javierm@redhat.com>
To:     Heiko =?utf-8?Q?St=C3=BCbner?= <heiko@sntech.de>,
        Peter Geis <pgwipeout@gmail.com>
Cc:     linux-kernel@vger.kernel.org,
        Peter Robinson <pbrobinson@gmail.com>,
        Caleb Connolly <kc@postmarketos.org>,
        Jarrah Gosbell <kernel@undef.tools>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Martijn Braam <martijn@brixit.nl>, Ondrej Jirman <megi@xff.cz>,
        Rob Herring <robh+dt@kernel.org>,
        Tom Fitzhenry <tom@tom-fitzhenry.me.uk>,
        devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-rockchip@lists.infradead.org
Subject: Re: [PATCH] arm64: dts: rockchip: Change serial baud rate for
 Pinephone Pro to 1.5 MB
In-Reply-To: <3999080.iIbC2pHGDl@diego>
References: <20230403175937.2842085-1-javierm@redhat.com>
 <3738011.44csPzL39Z@diego>
 <CAMdYzYqT=vVZxFK=P6xYDAht_KWk3CTsTtruPAbGqx1K9EcJsQ@mail.gmail.com>
 <3999080.iIbC2pHGDl@diego>
Date:   Tue, 18 Apr 2023 14:11:53 +0200
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Heiko St=C3=BCbner <heiko@sntech.de> writes:

Hello Heiko,

> Hi Peter,
>
> Am Dienstag, 4. April 2023, 14:52:02 CEST schrieb Peter Geis:
>> On Tue, Apr 4, 2023 at 3:55=E2=80=AFAM Heiko St=C3=BCbner <heiko@sntech.=
de> wrote:
>> >
>> > Hi,
>> >
>> > Am Montag, 3. April 2023, 19:59:37 CEST schrieb Javier Martinez Canill=
as:
>> > > This baud rate is set for the device by mainline u-boot and is also =
what
>> > > is set in the Pinebook Pro Device Tree, which is a device similar to=
 the
>> > > PinePhone Pro but with a different form factor.
>> > >
>> > > Otherwise, the baud rate of the firmware and Linux don't match by de=
fault
>> > > and a 'console=3DttyS2,1500000n8' kernel command line parameter is r=
equired
>> > > to have proper output for both.
>> >
>> > The interesting question is always if this will break someone else's s=
etup.
>> > I've never really understood the strange setting of 1.5MBps, but on the
>> > other hand it _is_ a reality on most boards.
>
>> The 1.5M baud is default because the clock structure on rockchip
>> devices does not allow a clean 115200 baud. By attempting to force
>> 115200, it will always be slightly off (either low or high depending
>> on how the driver decided to round). If this actually causes any
>> problems is the subject of much debate.
>
> thanks so much for this piece of clock-detail. As I wrote, I never really
> understood the why _before_ but also never cared that much to dive
> into it and find out.
>
> So your explanation closes one knowledge gap in my head.
>
> Thanks a lot :-)

Did you make a decision about this? I guess the clock explanation is yet
another argument in favour of switching the PPP to a 1.5 Mbps baud rate ?

--=20
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat