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

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Date:   Thu, 30 Jan 2020 17:11:55 +0100
From:   "Nicolas Saenz Julienne" <nsaenzjulienne@...e.de>
To:     "Lukas Wunner" <lukas@...ner.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 Tue Jan 28, 2020 at 3:19 PM, Lukas Wunner wrote:
> On Tue, Jan 28, 2020 at 01:42:21PM +0100, Nicolas Saenz Julienne wrote:
> > I'm testing this by booting directly from RPi4's bootloader. And it
> > works as long as I add this to config.txt:
> > 
> > enable_uart=1
> > gpu_freq=500
> > 
> > Which AFAIK blocks frequency scalin on the GPU and fixes the clock to a
> > point where the serial is set at 115200 bauds.
> > 
> > Ideally it'd be nice to be able to query the clock frequency, and
> > recalculate the divisors based on that. But I don't know if it's
> > feasible at that point in the boot process.
>
> Well, we don't even support adjusting the baudrate *after* the kernel
> has booted.

Don't 8250 uarts have a clk divisor? I've seen other 8250 earlycon
drivers do this (see 8250_ingenic.c), that said I think it's a lost
cause for us.

BTW did you had the oportunity to have a go at the patch?

> 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.

Regards,
Nicolas