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Date:   Tue, 4 Apr 2023 17:40:17 +0200
From:   =?utf-8?Q?Ond=C5=99ej?= Jirman <megi@xff.cz>
To:     Javier Martinez Canillas <javierm@redhat.com>
Cc:     Heiko =?utf-8?Q?St=C3=BCbner?= <heiko@sntech.de>,
        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>,
        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
Message-ID: <20230404154017.7n6rcgqyv3edfahd@core>
Mail-Followup-To: =?utf-8?Q?Ond=C5=99ej?= Jirman <megi@xff.cz>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Heiko =?utf-8?Q?St=C3=BCbner?= <heiko@sntech.de>,
        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>, 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
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On Tue, Apr 04, 2023 at 04:04:53PM +0200, Javier Martinez Canillas wrote:
> Ondřej Jirman <megi@xff.cz> writes:
> 
> > On Tue, Apr 04, 2023 at 09:51:11AM +0200, Heiko Stübner 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.
> >
> > Normal users of the phone probably run with UART console disabled, because
> > UART is muxed with audio jack output and to enable it they have to add
> > console=ttyS2 to the kernel command line and flip a physical switch inside
> > the phone.
> >
> > Fortunately, not sepcifying stdout-path baud rate in the options part
> > of the string, will make the serial driver probe for the baud rate from
> > the previous boot stage and make the user happy by keeping whatever was
> > already set in the bootloader.
> >
> 
> As mentioned in the first email of this thread, I tried that but it didn't
> work for me. Either using stdout-path = "serial2"; or stdout-path = &uart2;
> gives me no serial output with console=ttyS2, I needed to specify the baud
> rate explicitly (i.e: console=ttyS2,1500000n8).
> 
> >   https://elixir.bootlin.com/linux/latest/source/drivers/tty/serial/8250/8250_port.c#L3496
> >
> 
> Is that helper really used by the serial driver in the PinePhone Pro?
> (drivers/tty/serial/8250/8250_dw.c), I was meaning to dig why just ttyS2
> was not working but decided that could be a follow-up patch.

It's called by univ8250_console_setup in 8250_core.c But looking at it again,
this may only be used to match console=uart.

  https://elixir.bootlin.com/linux/latest/source/drivers/tty/serial/8250/8250_core.c#L638

Bummer.

> Since chaging to 1.5 MBps seemed to have merits on its own, decided to
> post this patch anyways in the meantime.
> 
> > So we can make the kernel just keep the baudrate setup from the previous
> > boot stage by:
> >
> > 	stdout-path = "serial2";
> >
> 
> Did it work for you? Maybe I'm doing something silly but as mentioned it
> didn't work for me with upstream u-boot.

I was just eyeballing the code. I didn't test it.

kind regards,
	o.

> > regards,
> > 	o.
> >
> 
> -- 
> Best regards,
> 
> Javier Martinez Canillas
> Core Platforms
> Red Hat
>