Garbage UART output on RPI 4 with upstream kernel DTB

[Matthias Brugger]

Hi Stefan,

sorry for the delay, answers inside.

On 25/12/2019 14:55, Stefan Wahren wrote:
> Am 24.12.19 um 13:38 schrieb Matthias Brugger:
>>
>> On 24/12/2019 12:56, Stefan Wahren wrote:
>>> Hi Matthias,
>>>
>>> Am 23.12.19 um 21:20 schrieb Matthias Brugger:
>>>> Hi Stefan,
>>>>
>>>> On 23/12/2019 19:51, Stefan Wahren wrote:
>>>>> Am 20.12.19 um 14:58 schrieb Stefan Wahren:
>>>>>> Hi,
>>>>>>
>>>>>> i tried to run current U-Boot (rpi_4_32b_defconfig) on my RPi 4 with
>>>>>> bcm2711-rpi-4-b.dtb from the upstream kernel. Unfortunately i only see
>>>>>> garbage on the debug UART (pin 14 & 15). Using the DTB from the
>>>>>> downstream kernel has a proper UART output. The config.txt contains
>>>>>> debug_uart=1
>>>>>>
>>>>>> I compared both and identified an offending Linux commit:
>>>>>> ARM: dts: bcm283x: Remove brcm,bcm2835-pl011 compatible
>>>>>>
>>>>>> Unfortunately reverting this patch still doesn't fix the issue. Any ideas?
>>>>> After hours of playing with the DTB, i finally found the reason why the
>>>>> upstream kernel DTB doesn't work with U-Boot on RPI 4. The DTS must be
>>>>> compiled with flag "-@".
>>>>>
>>>>> It isn't clear to me, why this is necessary but it would be nice to make
>>>>> U-Boot work without this.
>>>>>
>>>> Can you provide the exact command you are using?
>>> i didn't call dtc directly via command line. I just added this Makefile
>>> hack [1] to the Linux upstream tree. This will add a __symbols__ section
>>> to the DTB which is required for overlays. You can verify this with dtdiff.
>>>
>>> Since the upstream kernel boots without the __symbols__ section, i think
>>> this garbage output issue could be fixed in u-boot.
>>>
>> Little by little I grasp what you are doing. So you compile the DTB in the
>> kernel, then pass the blob to u-boot and add it with OF_EMBED? Is that correct?
> 
> I use a recent Raspbian image, replace the bcm2711 DTB with the upstream
> kernel ones (yes build with make dtbs) and kernel7.img with u-boot.bin
> (renamed all the other kernel binaries to make sure kernel7.img is
> really used). Also use a boot.scr from here, but not sure this have an
> influence [1]. I used the rpi_4_32b_defconfig without any modifications
> like OF_EMBED. I assume u-boot uses the FDT image passed by the
> Raspberry Pi bootloader.

Yes exactly, FDT image is taken from the bootloader.

Can you please double check your config.txt especially the force_turbo and
initial_trubo options.

I can see garbage on the serial console when the initial_turbo time is over
(when setting force_turbo=0).
If I set force_turbo=1 I don't observe this behavior.

Regards,
Matthias


> 
> [1] - https://andrei.gherzan.ro/linux/uboot-on-rpi/
> 
>>
>> Regards,
>> Matthias
>