Re: Issues attempting to use Raspberry Pi 4 serial console on mainline

From: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
To: Peter Robinson <pbrobinson@gmail.com>,
	Nathan Chancellor <natechancellor@gmail.com>
Cc: clang-built-linux@googlegroups.com,
	bcm-kernel-feedback-list@broadcom.com,
	linux-rpi-kernel@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: Issues attempting to use Raspberry Pi 4 serial console on mainline
Date: Wed, 22 Jul 2020 17:46:22 +0200	[thread overview]
Message-ID: <afe9b10adee7b6085d0e880be84bfa8446b2a6a6.camel@suse.de> (raw)
In-Reply-To: <CALeDE9NCjMzzhdwR=oMh4wYjZXf8ekTCi8t_DVq0PLw5xD+8aQ@mail.gmail.com>

Hi Nathan, Peter,

On Wed, 2020-07-22 at 15:41 +0100, Peter Robinson wrote:

[...]

> On Wed, Jul 22, 2020 at 2:27 PM Nicolas Saenz Julienne
> > For reference I just booted linux-next with this setup:
> > 
> > boot partition:
> >         ...Latest firmware files taken from the RPi firmware repo [1]...

Natan, it seems I lied here and been using an outdated firmware version. I've
just validated what Peter comments below, and the last working firmware release
with this setup is Feb12[1]. Given your use case I'd just use that and forget
about it.

> >         Image                   #Copied from linux build
> >         bcm2711-rpi-4-b.dtb     #Copied from linux build
> >         config.txt
> >         cmdline.txt
> > 
> > config.txt:
> >         kernel=Image
> >         enable_uart=1
> >         arm_64bit=1
> > 
> > cmdline.txt:
> >          console=tty console=ttyS1,115200 text root=/dev/nfs
> >          nfsroot=10.42.0.1:/home/nico/netboot/root,vers=3 rw ip=dhcp
> > rootwait
> >          elevator=deadline
> > 

[...]

> > 
> > The issue with the mini UART is its clock, which is derived from VPU's, which
> > is itself controlled by RPi's firmware. Changes might happen behind the
> > kernel's back, and the mini UART divisors will not be updated accordingly.
> > This is an area the we could do better, but no one found a good solution yet.
> > That said, for now, when using the upstream kernel, VPU's clock should be
> > stable as we forbid the firmware from performing frequency scaling on that
> > clock.
> 
> There has actually been a regression in the firmware here, prior to
> mid April if the enable_uart=1 I always had clean output on the serial
> console, since that date on the rpi3/3b+/4 I get a whole bunch of
> junk, revert to an older firmware (I've been using April 1st ) and it
> all goes back to being fine. I've not had time to actually report it
> yet, and it's still a problem with the latest firmware but a quick
> look it likes similar to this issue reported in late April:
> 
> https://github.com/raspberrypi/firmware/issues/1376

Thanks for pointing it out! I missed that bug report. I'll try to get the gist
of it.

Regards,
Nicolas

[1] https://github.com/raspberrypi/firmware/releases

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