[U-Boot] Raspberry Pi Compute Module 1 mini-UART

[U-Boot] Raspberry Pi Compute Module 1 mini-UART

[edrose]

Hi,

I'm working on a project that uses the Raspberry Pi Compute Module 1. Much
like the CM 3, the CM 1 has two UART interfaces - the main UART at ttyAMA0,
and a second "mini-UART" at ttyS0. The project uses the main UART since it
works better at high speeds, so the console has been bumped over to the
mini-UART. This works fine for kernel messages and as a TTY console for
logging in and interacting with the device, however I've been unable to get
u-boot to present it's console over the mini-UART.

I've worked deep enough into this to know that u-boot doesn't use the device
tree that is set-up in config.txt, so the parameters that I've put in there
to activate and remap the mini-UART (UART1 in dts files) to pins 40/41
doesn't apply to u-boot. I'm using the bcm2835-rpi-b dts file currently
since it's the best fit for the hardware, but it leaves the mini-UART port
disabled. I've tried playing around with the dts files to enable/remap the
mini-UART port however I've been unsuccessful.

Can anyone give me any pointers as to how I modify the dts files to enable
and map the second UART for u-boot to be able to use it?

Kindest regards,
Ed



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[U-Boot] Raspberry Pi Compute Module 1 mini-UART

[Matthias Brugger]

Hi Ed,

On 25/11/2019 21:47, edrose wrote:
> Hi,
> 
> I'm working on a project that uses the Raspberry Pi Compute Module 1. Much
> like the CM 3, the CM 1 has two UART interfaces - the main UART at ttyAMA0,
> and a second "mini-UART" at ttyS0. The project uses the main UART since it
> works better at high speeds, so the console has been bumped over to the
> mini-UART. This works fine for kernel messages and as a TTY console for
> logging in and interacting with the device, however I've been unable to get
> u-boot to present it's console over the mini-UART.
> 
> I've worked deep enough into this to know that u-boot doesn't use the device
> tree that is set-up in config.txt, so the parameters that I've put in there
> to activate and remap the mini-UART (UART1 in dts files) to pins 40/41
> doesn't apply to u-boot. I'm using the bcm2835-rpi-b dts file currently
> since it's the best fit for the hardware, but it leaves the mini-UART port
> disabled. I've tried playing around with the dts files to enable/remap the
> mini-UART port however I've been unsuccessful.
> 
> Can anyone give me any pointers as to how I modify the dts files to enable
> and map the second UART for u-boot to be able to use it?
> 

Basically there are two ways.

1) as you found out U-Boot is using an embedded device tree. The device tree it
uses is specified in configs/rpi_*
I suppose you are using rpi_3_defconfig, so your device-tree is: bcm2837-rpi-3-b
You can find the device-tree files in arch/arm/dts where you can change the files.

2) you can update the config to use CONFIG_OF_BOARD instead of CONFIG_OF_EMBED.
This will take the device-tree from the file specified in config.txt (or to be
correct the RPi FW will pass this device-tree to U-Boot in a register at startup).

Good luck!

Matthias

> Kindest regards,
> Ed
> 
> 
> 
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[U-Boot] Raspberry Pi Compute Module 1 mini-UART

[edrose]

Hi Matthias,

Thank you very much for your help. Unfortunately I'm still a little stuck.


Matthias Brugger wrote
> 1) as you found out U-Boot is using an embedded device tree. The device
> tree it
> uses is specified in configs/rpi_*
> I suppose you are using rpi_3_defconfig, so your device-tree is:
> bcm2837-rpi-3-b
> You can find the device-tree files in arch/arm/dts where you can change
> the files.

I had a go at this, but with little success. The Compute Module 1 is
actually based off the hardware of the original 256Mb Raspberry Pi 1. I've
tried using the bcm2837-rpi-3-b device-tree, but I just get (what I've come
to call) the Rainbow Screen of Death and nothing boots. Using the
bcm2835-rpi-b tree allows it to boot.

I tried modifying the file to include the second UART port, by adding the
following lines to the end (underneath the definition for &uart0), but
unfortunately it doesn't seem to work:

&uart1 {
    pinctrl-names = "default";
    pinctrl-0 = <&uart1_gpio40>;
    status = "okay";
};

I'm testing this by calling `coninfo` in the uboot prompt. Two serial ports
show, however only one has an address next to it and it's the address of the
main serial port. It looks like the following:

List of available devices:                                                     
serial at 7e201000 00000007 IO                                                    
serial   00000003 IO stdin                                                     
nulldev  00000003 IO                                                           
vidconsole 00000002 .O stdout stderr

Am I interpreting that correctly? Or is the serial port somehow enabled
without an address?


'Matthias Brugger" wrote
> 2) you can update the config to use CONFIG_OF_BOARD instead of
> CONFIG_OF_EMBED.
> This will take the device-tree from the file specified in config.txt (or
> to be
> correct the RPi FW will pass this device-tree to U-Boot in a register at
> startup). 

Doing this (but leaving the default device tree the same) results in no
changes to the output of `coninfo`. Is there something I need to do to tell
uboot where in memory the device-tree is stored?

I've also tried playing around with CONFIG_SPECIFY_CONSOLE_INDEX but that
had no effect either. Is there something that needs adding to the header
file to specify the serial port?

Thanks once again for your help,
Ed



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[U-Boot] Raspberry Pi Compute Module 1 mini-UART

[edrose]

I've done some more digging, and have made a bit of progress (I think).

After fiddling with the device-tree, I've managed to get the serial port to
show up when running `dm tree`. I appended this block to the end of
bcm2835-rpi-b.dts:

&uart1 {
	pinctrl-names = "default";
	pinctrl-0 = <&uart1_gpio40>;
	status = "okay";
};

And the relevant output of `dm tree is:

Class    index  Probed  Driver      Name                                       
-----------------------------------------    
                          ...
pinconfig   32  [ + ]   pinconfig   |   |   |-- uart0_gpio14
                          ...
pinconfig   42  [   ]   pinconfig   |   |   |-- uart1_gpio40
                          ...
serial      0  [ + ]   bcm283x_pl  |   |-- serial at 7e201000                     
serial      1  [   ]   serial_bcm  |   |-- serial at 7e215040                     

So that indicates that it sees the serial port, knows which driver to use,
and has the correct address. However it has not probed the device, nor has
it probed the pinconfig that MUXes it to the pin I've set in the dts file.

It also appears to have an index , however specifying CONFIG_CONS_INDEX
doesn't change the port that is used, nor does it change the output of
`dm-tree`.

So I'm still a bit stuck. How do I force uboot to use a different (already
known) serial port for the console? The drivers are there, it sees the port,
it just doesn't want to use it...



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Raspberry Pi Compute Module 1 mini-UART

[edrose]

I've done a little more work on this. I've been able to swap the UART pins
around to a certain extent, but it's still being stubborn. I've been able to
move the console output from the main UART from GPIO14 to GPIO32. I've
somehow also managed to get the main UART to output to both GPIO32 and
GPIO14 simultaneously (although it wouldn't RX anything when that was
happening)! However I still can't get it to simply move the console over to
using the mini-UART.

I've worked out that the serial port seems to be selected by the status of
the pin multiplexers. It checks to see whether GPIO15 is multiplexed to the
main port. If not, it checks to see whether GPIO15 is multiplexed to the
alternate port. I tried forcing it to think that the secondary UART was
multiplexed by forcing a return value in the is_serial_muxed() functions to
be false for the main UART and true for the mini-UART, however it just gives
me a rainbow splash screen of death.

Can I have some pointers as to how I debug what's causing the Pi to fail to
boot when I try forcing the mini-UART? Clearly something is wrong and
causing it to fail. Will it output anything over the early debug UART and,
if so, what values do I need to put into the config for that to work? I
tried adding the register addresses I found in the datasheet, however I
couldn't get anything out of it.

Kindest regards,
Ed



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