linux-kernel - Re: [PATCH] arm64: dts: rockchip: Change serial baud rate for Pinephone Pro to 1.5 MB

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Date:   Tue, 18 Apr 2023 14:11:53 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Heiko Stübner <heiko@...ech.de>,
        Peter Geis <pgwipeout@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Robinson <pbrobinson@...il.com>,
        Caleb Connolly <kc@...tmarketos.org>,
        Jarrah Gosbell <kernel@...ef.tools>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Martijn Braam <martijn@...xit.nl>, Ondrej Jirman <megi@....cz>,
        Rob Herring <robh+dt@...nel.org>,
        Tom Fitzhenry <tom@...-fitzhenry.me.uk>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH] arm64: dts: rockchip: Change serial baud rate for
 Pinephone Pro to 1.5 MB

Heiko Stübner <heiko@...ech.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 AM Heiko Stübner <heiko@...ech.de> wrote:
>> >
>> > Hi,
>> >
>> > Am Montag, 3. April 2023, 19:59:37 CEST schrieb Javier Martinez Canillas:
>> > > 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 default
>> > > and a 'console=ttyS2,1500000n8' kernel command line parameter is required
>> > > to have proper output for both.
>> >
>> > The interesting question is always if this will break someone else's setup.
>> > 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 ?

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat