enable SCD4x as module

[schoerg]

Describe the bug

Currently, sensors like the SCD41 are not usable without some user-space program. It uses I2C address 0x62

[03:02:54] root@raspberrypi:/# modprobe scd4x
modprobe: FATAL: Module scd4x not found in directory /lib/modules/6.12.51-v8-16k+

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: UU -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- 62 -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/iio/chemical/scd4x.c?h=v6.18-rc1

Manufacturer: https://sensirion.com/products/catalog/SCD41

Steps to reproduce the behaviour

[03:02:54] root@raspberrypi:/# modprobe scd4x
modprobe: FATAL: Module scd4x not found in directory /lib/modules/6.12.51-v8-16k+

Kernel build config:

# Chemical Sensors
#
[...]
# CONFIG_SCD4X is not set
[...]

Device (s)

Raspberry Pi 5

System

Raspberry Pi reference 2025-10-01
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 7dadcf1fc5ce1648ab09409ab978831690c9a955, stage2

2025/10/08 17:19:18
Copyright (c) 2012 Broadcom
version a06c733f (release) (embedded)

Linux raspberrypi 6.12.51-v8-16k+ #1910 SMP PREEMPT Mon Oct  6 13:45:09 BST 2025 aarch64 GNU/Linux

Logs

No response

Additional context

No response

[pelwell]

#7094 implements the support you have asked for. In about half an hour you should be able to install a trial image using sudo rpi-update pulls/7094. Back up any important data first, and you may want to test that your system still boots with auto_initramfs=0 in config.txt, otherwise updating the kernel in this way could leave you with a non-booting Pi.

[schoerg]

Thank you!

[popcornmix]

Thank you!

Does this mean you've tested it and it worked?

[schoerg]

Update installation worked, but loading somewhat not,

in config.txt:

dtoverlay=scd4x
but i2cdetect -y 1 still shows no loaded kernel module:

root@raspberrypi:~# i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: UU -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- 62 -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

If I load it manually with

modprobe scd4x 
echo scd4x 0x62 > /sys/bus/i2c/devices/i2c-1/new_device

dmesg shows that it's loaded

[95997.430356] i2c i2c-1: new_device: Instantiated device scd4x at 0x62

But same output in i2cdetect as before, so I don't know if this is a issue specific with the Rpi kernel.

[pelwell]

I should have made it clear that this change extends the i2c-sensor overlay to support the SCD4X. Try:

dtoverlay=i2c-sensor,scd4x

[pelwell]

(Comment updated to show that the scd4x parameter should not be capitalised, despite what the autocorrect on my phone thought)

[schoerg]

Thanks, works now!

drwxr-xr-x 2 root root   0 Oct 19 13:13 buffer
drwxr-xr-x 2 root root   0 Oct 19 13:13 buffer0
-rw-r--r-- 1 root root 16K Oct 19 13:13 calibration_auto_enable
--w------- 1 root root 16K Oct 19 13:13 calibration_forced_value
-r--r--r-- 1 root root 16K Oct 19 13:13 calibration_forced_value_available
-r--r--r-- 1 root root 16K Oct 19 13:13 dev
-rw-r--r-- 1 root root 16K Oct 19 13:13 in_concentration_co2_raw
-rw-r--r-- 1 root root 16K Oct 19 13:13 in_concentration_co2_scale
-rw-r--r-- 1 root root 16K Oct 19 13:13 in_humidityrelative_raw
-rw-r--r-- 1 root root 16K Oct 19 13:13 in_humidityrelative_scale
-rw-r--r-- 1 root root 16K Oct 19 13:13 in_temp_calibbias
-rw-r--r-- 1 root root 16K Oct 19 13:13 in_temp_offset
-rw-r--r-- 1 root root 16K Oct 19 13:13 in_temp_raw
-rw-r--r-- 1 root root 16K Oct 19 13:13 in_temp_scale
-r--r--r-- 1 root root 16K Oct 19 13:13 name
lrwxrwxrwx 1 root root   0 Oct 19 13:13 of_node -> ../../../../../../../../firmware/devicetree/base/axi/pcie@1000120000/rp1/i2c@74000/scd4x@62
-rw-r--r-- 1 root root 16K Oct 19 13:13 out_pressure_raw
-r--r--r-- 1 root root 16K Oct 19 13:13 out_pressure_raw_available
drwxr-xr-x 2 root root   0 Oct 19 13:13 power
drwxr-xr-x 2 root root   0 Oct 19 13:13 scan_elements
lrwxrwxrwx 1 root root   0 Oct 19 13:13 subsystem -> ../../../../../../../../bus/iio
drwxr-xr-x 2 root root   0 Oct 19 13:13 trigger
-rw-r--r-- 1 root root 16K Oct 19 13:13 uevent
-r--r--r-- 1 root root 16K Oct 19 13:13 waiting_for_supplier

[pelwell]

Thanks for reporting back. The PR has now been merged, and the updated overlay will appear in future kernel releases.