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From:   Heiko Stuebner <heiko@sntech.de>
To:     Peter Geis <pgwipeout@gmail.com>,
        Javier Martinez Canillas <javierm@redhat.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
Date:   Fri, 28 Jul 2023 20:59:50 +0200
Message-ID: <4495367.TLkxdtWsSY@phil>
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References: <20230403175937.2842085-1-javierm@redhat.com> <3797122.KgjxqYA5nG@diego>
 <87pm4kuanl.fsf@minerva.mail-host-address-is-not-set>
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Hi Javier,

Am Samstag, 22. Juli 2023, 01:06:54 CEST schrieb Javier Martinez Canillas:
> Heiko Stübner <heiko@sntech.de> writes:
> > Am Dienstag, 18. April 2023, 14:11:53 CEST schrieb Javier Martinez Canillas:
> >> Heiko Stübner <heiko@sntech.de> writes:
> >> > Am Dienstag, 4. April 2023, 14:52:02 CEST schrieb Peter Geis:
> >> >> On Tue, Apr 4, 2023 at 3:55 AM Heiko Stübner <heiko@sntech.de> wrote:
> >> >> > 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 ?
> >
> > Sorry, but no decision made here. Either way it's breaking for someone,
> > which makes this quite hard.
> >
> 
> Another ping on this patch.
> 
> > The rate accuracy is the one side, the two-boot issue is the other side.
> > And mainline u-boot (and levinboot - whatever that is) provides a 3rd side.
> >
> > People starting with the phone probably won't replace the bootloader
> > in a first step but instead might play with a system image or newer kernel.
> > So if the uart will break for everyone using the default bootloader from
> > the factory that is somewhat bad.
> >
> 
> Probably won't replace the DTB shipped with the firmware either? If one is
> replacing the firmware provided DTB witch the one in the mainline kernel,
> probably such person is also using mainline u-boot?

Not necessarily.

I.e. putting an extlinux.conf on an sd-card with a kernel-image and dtb
is not rocket science ;-)


> > I don't have a Pinephone Pro myself, so I really hoped for some Acks
> > or similar to appear in the meantime.
> >
> 
> For someone like me who is only using mainline u-boot, linux, etc then
> having a consistent uart baud rate across all components is really useful.
> 
> Otherwise I either have serial console for u-boot or the kernel, but can't
> have both working so is annoying.
> 
> It would be good to have a definite answer on this. Since every time that
> I try to hack on my PPP, I end changing my DTS and remember this patch :)

So far people only reported "breaks my setup". I'm in a pickle here ;-) .
Without anybody saying "I want to also move into this direction" I really
feel I should not merge a patch that breaks other peoples setups.


Heiko