linux-kernel - Re: [PATCH] serial: 8250_early: Add earlycon for BCM2835 aux uart

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Date:   Fri, 31 Jan 2020 16:24:14 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     Matthias Brugger <matthias.bgg@...il.com>, matthias.bgg@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        Matthias Brugger <mbrugger@...e.com>,
        Scott Branden <sbranden@...adcom.com>,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        Ray Jui <rjui@...adcom.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org, linux-serial@...r.kernel.org,
        jslaby@...e.com
Subject: Re: [PATCH] serial: 8250_early: Add earlycon for BCM2835 aux uart

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?

I've just performed a quick test and it doesn't work for me.
If I add stdout-path = "serial1:115200n8"; to the chosen node,
I only get a regular console with this patch, not an earlycon.


> > The problem is that in mainline, bcm2835_defconfig contains:
> > CONFIG_CPU_FREQ_DEFAULT_GOV_CONSERVATIVE=y
> >
> > Likewise in the Foundation's downstream tree, bcmrpi_defconfig as well
> > as bcm2711_defconfig and bcm2709_defconfig contain:
> > CONFIG_CPU_FREQ_DEFAULT_GOV_POWERSAVE=y
> >
> > In contrast to this, we set the following on Revolution Pi devices:
> > CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE=y
> >
> > 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.
> 
> 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.

Okay, I'll take a look.  But the uart1 baudrate will still be wrong
if the firmware downclocks because of overheating, right?

Thanks,

Lukas