feat: kernel module

pisugar-module/Makefile

@@ -1,10 +1,14 @@
+TARGET = pisugar_3_battery
+
 obj-m += pisugar_3_battery.o
 
-KERN_VER=$(shell uname -r)
+KERN_VER = $(shell uname -r)
+
+SOURCEDIR = $(shell pwd)
+BUILDDIR = $(shell pwd)/build
 
 all:
-	make -C /lib/modules/$(KERN_VER)/build M=$(shell pwd) modules
+	make -C /lib/modules/$(KERN_VER)/build M=$(SOURCEDIR) modules
 
 clean:
-	rm -f *.cmd *.ko *.o Module.symvers modules.order *.mod.c
-
+	rm -f *.cmd *.ko *.o Module.symvers modules.order *.mod.c .pisugar_3_battery.* .modules.* .Module.symvers.* pisugar_3_battery.mod

pisugar-module/README.md

@@ -1,14 +1,63 @@
 # PiSugar Linux kernel modules
 
-Make kernel modules:
+Linux kernel modules for PiSugar 3.
+
+## Preparing for building RPI kernel module
+
+### Linux distributions with kernel symbols
+
+Congratulations if your PI is running a linux distribution that has `/lib/modules/$(uname -r)/build/` directory, e.g. ubuntu-server, you don't need to manually build RPI kernel, and that will save a lot of time.
+
+Install linux-headers
+```shell
+sudo apt install linux-headers-$(uname -r)
+```
+
+### Raspberry Pi OS
+
+As kernel symbols is not included in Raspberry Pi OS (no `/lib/modules/$(uname -r)/build`), so you need to compile the kernel and generate the kernel symbols by youself. 
+
+To build the kernel, see official doc: https://www.raspberrypi.com/documentation/computers/linux_kernel.html
+
+** It seems like the precompiled symbols could be downloaded from [here](https://github.com/raspberrypi/firmware), but I could not figure out the correct steps. **
+
+First, get RPI OS tag, e.g. 1.20230405
+```shell
+dpkg -l | grep kernel
+```
+
+Clone kernel repository, and create a symbol link
+```shell
+git clone --depth 1 --branch <PI_OS_TAG> https://github.com/raspberrypi/linux.git
+sudo ln -s "$(pwd)/linux" /usr/src/linux
+```
+
+Build RPI kernel.
+
+When it is done, copy and rename `linux` folder to PI `/lib/modules/$(uname -r)/build`.
+
+## Compiling kernel module
+
+Clone this repository, make kernel modules:
 
     make
 
 Load module:
 
     sudo insmod pisugar_3_battery.ko
+    # or
+    sudo insmod pisugar_3_battery.ko i2c_bus=1 i2c_addr=0x57
+
+Then you will see extra battry files in `/sys/class/power_supply`
+
+    ls /sys/class/power_supply
 
 Remove module:
 
     sudo rmmod pisugar_3_battery.ko
 
+Now, you can enable [battery monitor plugin](https://github.com/raspberrypi-ui/lxplug-ptbatt).
+
+## License
+
+GPL