RPi3: serial console

[Stefan Agner]

On 2020-10-16 03:28, Ricardo Salveti wrote:
> On Tue, Oct 13, 2020 at 11:04 AM Peter Robinson <pbrobinson at gmail.com> wrote:
>>
>> > >> Hello Matthias,
>> > >>
>> > >> I have upgraded the Raspberry 3 firmware from raspi3-firmware
>> > >> (1.20190215-1+deb10u4) to raspi-firmware (1.20200601-3)
>> > >> [https://packages.debian.org/bullseye/raspi-firmware].
>> > >>
>> > >> After the upgrade the output of U-Boot on the serial console is complete
>> > >> gibberish as if the baudrate were incorrect. The output from the Linux
>> > >> kernel is fine at 115200 baud.
>> > >
>> > > I've seen similar on all firmware since around mid April up until
>> > > recently, it seems to be fixed with releases in Oct (I'm using one
>> > > from Oct 8th), I'm not sure if it can be fixed in U-Boot but it seems
>> > > to be due to a change in the firmware.
>> >
>> > Thanks for confirming the problem.
>>
>> For reference I have found a number of other problems with recent
>> firmwares and 2020.10 release:
>> * U-Boot crashes on a RPi4 8Gb model if you don't have a display connected [1]
>> * It doesn't boot, not sure if it's a crash or something else, you
>> just get the rainbow screen, if you don't have the uart enabled in
>> config.txt (ie just using a display for output).
> 
> Noticed the same as well here (boots fine only with enable_uart=1).
> While playing over with the files I noticed it works fine with the
> latest firmware when using an older bcm2710-rpi-3-b-plus.dtb
> (placed together with the firmware). If I use the latest
> bcm2710-rpi-3-b-plus.dtb from the 5.4 downstream kernel, it doesn't
> work (only tested on rpi3-b-plus).
> 
> This is with CONFIG_OF_BOARD=y, if I use CONFIG_OF_EMBED instead it
> works just fine (which gives me the impression that u-boot might not
> be happy with the dtb generated by the firmware when enable_uart is
> not set).

What we noticed is adding brcm,bcm2835-pl011 compatible to the UARTs
makes U-Boot 2020.10 boot even *without* enable_uart=1. dm shows that
with this change CONFIG_BCM283X_PL011_SERIAL is in use...

>From what I understand, this driver checks if the UART is muxed, and if
not bails out. So I guess without that compatible string, when the
regular PL011 driver is used, it seems that U-Boot tries to initialize
UART and fails?

--
Stefan