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

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Message-ID: <nk22axbl47myincpxnhcygtb5viyldv6fkxng3m43yd7dwbzau@p6ycsad6uoqr>
Date:   Fri, 28 Jul 2023 22:10:36 +0200
From:   Ondřej Jirman <megi@....cz>
To:     Heiko Stuebner <heiko@...ech.de>
Cc:     Peter Geis <pgwipeout@...il.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        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>,
        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

Hi Heiko and Javier,

On Fri, Jul 28, 2023 at 08:59:50PM +0200, Heiko Stuebner wrote:
> 
> Hi Javier,
> 
> > 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 ;-)
> 
> > 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.

Even if this is not changed in kernel, bootloader should fix this situation by
patching the kernel DTB before booting the kernel. U-Boot already patches the
DTB in several ways, so I guess it's possible to just change stdout-path either
in generic code, or in per-board ft_board_setup() for Pinephone Pro to something
that matches whatever the user configured via Kconfig:

https://elixir.bootlin.com/u-boot/latest/source/configs/pinephone-pro-rk3399_defconfig#L79

I mean that if U-Boot allows the user to configure arbitrary baudrate via
KConfig, then it *should* also update the kernel DTB to match, especially if the
choice is different from DTB that it actually embeds:

https://elixir.bootlin.com/u-boot/latest/source/arch/arm/dts/rk3399-pinephone-pro.dts#L29

No static baudrate choice in DT will save anyone anyway, with the provided flexibility
of configuration.

Maybe this would be a nice location to fix this:

https://elixir.bootlin.com/u-boot/v2023.07.02/source/common/fdt_support.c#L131

kind regards,
	o.

> 
> Heiko
> 
>