Shell does not work on nrf54l15bsim using zephyr,native-pty-uart

[JaagupAverin]

Describe the bug

Hey! I am trying to get the BabbleSim simulation working with both console and shell. While the former worked out of the box, the latter does not.

I have created and chosen the native-pty-uart as such:

    chosen {
        zephyr,console = &uart_pty_0;
        zephyr,shell-uart = &uart_pty_0;
    };

    uart_pty_0: uart_pty_0 {
        compatible = "zephyr,native-pty-uart";
        status = "okay";
    };

Enabled logging and shell as such:

CONFIG_UART_NATIVE_PTY_0_ON_OWN_PTY=y

# =============================================================================
# Console:

CONFIG_LOG=y
CONFIG_SERIAL=y
CONFIG_UART_CONSOLE=y
CONFIG_LOG_BACKEND_UART=y
CONFIG_LOG_MODE_DEFERRED=y
CONFIG_LOG_BUFFER_SIZE=4096

# =============================================================================
# Shell:

# Basics:
CONFIG_SHELL=y
CONFIG_SHELL_TAB=y
CONFIG_SHELL_CMDS=y
CONFIG_SHELL_HISTORY=y

# Backends:
CONFIG_SHELL_LOG_BACKEND=y
CONFIG_SHELL_BACKEND_SERIAL=y

# Shell features:
CONFIG_KERNEL_SHELL=y
CONFIG_DEVICE_SHELL=y

And the application runs a simple hello world loop:

int main() {
    LOG_INF("Hello from HMI; " APP_VERSION_STRING);

    while (true) {
        LOG_INF("TICK!");
        k_sleep(K_MSEC(1000));
    }
    return 0;
}

Connecting to the pty port, no shell prompt is printed, and keystrokes are not echoed back nor do anything, while the console works as expected.

Regression

• This is a regression.

Steps to reproduce

No response

Relevant log output

Running the babblesim zephyr.exe prints:

zephyr.exe --nosim --wait_uart
d_00: @00:00:00.000000 WARNING: (/home/jaagup/projects/gale_ws/gale/projects/modules/bsim_hw_models/nrf_hw_models/src/HW_models/NHW_GRTC.c:441): The RT counter was not started while trying to read SYSCOUNTERL
d_00: @00:00:00.000000 WARNING: (/home/jaagup/projects/gale_ws/gale/projects/modules/bsim_hw_models/nrf_hw_models/src/HW_models/NHW_GRTC.c:441): The RT counter was not started while trying to read SYSCOUNTERH
d_00: @00:00:00.000000  uart_pty_0 connected to pseudotty: /dev/pts/16

---

Connecting to the pts port prints:

picocom v3.1

port is        : /dev/pts/16
flowcontrol    : none
baudrate is    : 115200
parity is      : none
databits are   : 8
stopbits are   : 1
escape is      : C-a
local echo is  : no
noinit is      : no
noreset is     : no
hangup is      : no
nolock is      : no
send_cmd is    : sz -vv
receive_cmd is : rz -vv -E
imap is        : 
omap is        : 
emap is        : crcrlf,delbs,
logfile is     : none
initstring     : none
exit_after is  : not set
exit is        : no

Type [C-a] [C-h] to see available commands
Terminal ready
*** Booting My Application v0.1.0-alpha-a0a6a6afd2d5 ***
*** Using nRF Connect SDK v3.0.99-e612a113ce24 ***
*** Using Zephyr OS v4.1.99-1e9016c290bc ***
[00:00:00.000,093] <inf> main: Hello from HMI; 0.1.0-alpha
[00:00:00.000,093] <inf> main: TICK!
[00:00:01.000,100] <inf> main: TICK!

Impact

Functional Limitation – Some features not working as expected, but system usable.

Environment

• Ubuntu 25.04
• Zephyr 1e9016c290bc7e2e14445b77520fbab979f93e63
• BabbleSim, nrf54l15bsim board

Additional Context

No response

[aescolar]

(Converting this bug report to a discussion, as it is more of a request for support than a bug report.)

@JaagupAverin Any reason to not use directly the nRF HW (modeled) UART?

Creating a DT overlay to set the chosen uart for the shell
samples/subsys/shell/shell_module/boards/nrf54l15bsim_nrf54l15_cpuapp.overlay :

/ {
	chosen {
		zephyr,shell-uart = &uart20;
	};
};

Things should work directly, or?

zephyr$ mkdir build && cd build
build$ cmake -GNinja -DBOARD=nrf54l15bsim/nrf54l15/cpuapp ../samples/subsys/shell/shell_module/ && ninja
build$ zephyr/zephyr.exe --uart_list
UART #: UART name
     0: UARTE00
     1: UARTE20
     2: UARTE21
     3: UARTE22
     4: UARTE30
build$ zephyr/zephyr.exe --uart1_pty_attach

[JaagupAverin]

I initially had issues with redirecting the modeled UART away from the main executable terminal, but I got it working as wanted using native-pty-uart, so I simply stuck with the latter.
With some new wisdom I can produce the wanted setup with the modeled UART as well.

[JaagupAverin]

There is a downside to using the HW modeled UART for me:

I use the babblesim handbrake device, which I suspect causes delays with the user input from the shell (I believe the device is suspended by the handbrake device, during which time all keystrokes are stalled) and during this stall the RX buffer tends to overflow quite a bit:

d_00: @00:06:53.018357 WARNING: (/home/jaagup/projects/gale_ws/gale/projects/modules/bsim_hw_models/nrf_hw_models/src/HW_models/NHW_UART.c:357): UART1: Pushed to full Rx FIFO, oldest value dropped
d_00: @00:06:53.018444 WARNING: (/home/jaagup/projects/gale_ws/gale/projects/modules/bsim_hw_models/nrf_hw_models/src/HW_models/NHW_UART.c:357): UART1: Pushed to full Rx FIFO, oldest value dropped
d_00: @00:06:54.019140 WARNING: (/home/jaagup/projects/gale_ws/gale/projects/modules/bsim_hw_models/nrf_hw_models/src/HW_models/NHW_UART.c:357): UART1: Pushed to full Rx FIFO, oldest value dropped

[aescolar]

Anyhow, if you really want to use the zephyr,native-pty-uart with the nrf54l15bsim, you should also be able to do so:
This would seem equivalent to what you are trying to do(?):
samples/subsys/shell/shell_module/boards/nrf54l15bsim_nrf54l15_cpuapp.overlay:

/ {
    chosen {
        zephyr,console = &uart_pty_0;
        zephyr,shell-uart = &uart_pty_0;
    };

    uart_pty_0: uart_pty_0 {
        compatible = "zephyr,native-pty-uart";
        status = "okay";
    };
};

&uart20 {
   status = "disabled";
};

diff --git a/samples/subsys/shell/shell_module/prj.conf b/samples/subsys/shell/shell_module/prj.conf
index ff7c665b381..438eae99ea1 100644
--- a/samples/subsys/shell/shell_module/prj.conf
+++ b/samples/subsys/shell/shell_module/prj.conf
@@ -16,3 +16,8 @@ CONFIG_THREAD_RUNTIME_STATS=y
 CONFIG_THREAD_RUNTIME_STATS_USE_TIMING_FUNCTIONS=y
 CONFIG_STATS=y
 CONFIG_STATS_SHELL=y
+
+CONFIG_UART_CONSOLE=y
+CONFIG_LOG_BACKEND_UART=y
+CONFIG_LOG_MODE_DEFERRED=y
+CONFIG_LOG_BUFFER_SIZE=4096

build$ cmake -GNinja -DBOARD=nrf54l15bsim/nrf54l15/cpuapp ../samples/subsys/shell/shell_module/ && ninja
build$ zephyr/zephyr.exe --attach_uart

[JaagupAverin]

Disabling uart20 does "fix" the shell not working not native-pty-uart ports. Thanks!

[aescolar]

Sidetrack, but just to clarify this. Note that the Babblesim simulation design is focused on speed and running tests in batch. Interactive runs are possible but can be quite clumsy (as you just found with the UART input overflowing)

I use the babblesim handbrake device, which I suspect causes delays with the user input from the shell (I believe the device is suspended by the handbrake device, during which time all keystrokes are stalled)

Yes, the handbrake device does indeed stall and release the whole simulation periodically.
You can adjust how often it does it with its --poke_period command line option (it can be lowered for a "smoother" run at the expense of more overhead).
But more importantly the zephyr bsim device does not wait for the physical layer side unless it needs to, or it hasn't for a very long time. How long is that long time can be set with the --mro option which by default is 1 second of simulated time.

[JaagupAverin]

Thanks a lot for these insights!
Main goal of using bsim in the long run is indeed running tests. But short term I don't have the actual board to work on yet so it'll work as an alternative :)
However I was also hopeful it could be a more comfortable alternative for developing bussiness logic code, as the development cycle is a bit faster (no programming delays) and its a bit more accessible (no board or remote tunneling needed).

[aescolar]

However I was also hopeful it could be a more comfortable alternative for developing bussiness logic code,

It is certainly possible, and many use it for this purpose.