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Message-ID: <dfbe2c94dd52a31826be751f8dd9afc4ed08ec6d.camel@suse.de>
Subject: Re: [PATCH] serial: 8250_early: Add earlycon for BCM2835 aux uart
From:   Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
To:     Lukas Wunner <lukas@wunner.de>
Cc:     Matthias Brugger <matthias.bgg@gmail.com>, matthias.bgg@kernel.org,
        linux-arm-kernel@lists.infradead.org,
        Matthias Brugger <mbrugger@suse.com>,
        Scott Branden <sbranden@broadcom.com>,
        gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
        Ray Jui <rjui@broadcom.com>,
        Stephen Boyd <swboyd@chromium.org>,
        Florian Fainelli <f.fainelli@gmail.com>,
        bcm-kernel-feedback-list@broadcom.com,
        linux-rpi-kernel@lists.infradead.org, linux-serial@vger.kernel.org,
        jslaby@suse.com
Date:   Mon, 03 Feb 2020 20:10:21 +0100
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On Fri, 2020-01-31 at 16:24 +0100, Lukas Wunner wrote:
> On Thu, Jan 30, 2020 at 05:11:55PM +0100, Nicolas Saenz Julienne wrote:
> > BTW did you had the oportunity to have a go at the patch?
>=20
> I've just performed a quick test and it doesn't work for me.
> If I add stdout-path =3D "serial1:115200n8"; to the chosen node,
> I only get a regular console with this patch, not an earlycon.

That's surprising, you're using u-boot isn't it? and the upstream device tr=
ee?

> > > The problem is that in mainline, bcm2835_defconfig contains:
> > > CONFIG_CPU_FREQ_DEFAULT_GOV_CONSERVATIVE=3Dy
> > >=20
> > > Likewise in the Foundation's downstream tree, bcmrpi_defconfig as wel=
l
> > > as bcm2711_defconfig and bcm2709_defconfig contain:
> > > CONFIG_CPU_FREQ_DEFAULT_GOV_POWERSAVE=3Dy
> > >=20
> > > In contrast to this, we set the following on Revolution Pi devices:
> > > CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE=3Dy
> > >=20
> > > Downclocking influences not only the uart1 baud rate but also the
> > > spi0 clock. We attach Ethernet chips to spi0, throughput was
> > > significantly worse with the ondemand governor (which is what we
> > > used previously). We felt that maximum Ethernet performance
> > > outweighs the relatively small powersaving gains.
> >=20
> > In that regard I suggest you use the upstream cpufreq driver which
> > behaves properly in that regard. It disables GPU freq scaling, so as to
> > change CPU frequencies without SPI/I2C/UART issues.
>=20
> Okay, I'll take a look.  But the uart1 baudrate will still be wrong
> if the firmware downclocks because of overheating, right?

You're right, it might happen. The way I understand it you're bound to leav=
e
the GPU at it's lower frequency if you want to use those peripherals and fo=
r
them to be reliable. You could technically try to empirically fine tune the=
 CPU
thermal trip point so as to make sure the upstream kernel cpufreq downclock
always kicks in before videocore's one. I'd actually like to see someone tr=
y
that. However short of using an RT kernel It think you'll never be 100% sur=
e it
can never happen.

Regards,
Nicolas


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