Issues attempting to use Raspberry Pi 4 serial console on mainline

[Nicolas Saenz Julienne]

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Natan, it seems I lied here and been using an outdated firmware version.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