Added tutorial how to compile kernel with can-j1939 module (#246)

Add tutorial how to compile kernel with can-j1939 module

can-j1939-install-kernel-module.md

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+# can-j1939 kernel module installation #
+
+
+
+### Problem
+
+You already have **can0** or **vcan0** up and working, **can-utils** downloaded and compiled to **~/can/can-utils** and you can send and receive frames without problems. However, when you want to bring up **can-j1939** you get error like this:
+
+```bash
+avra@vm-debian:~/can/can-utils$ sudo modprobe can-j1939
+modprobe: FATAL: Module can-j1939 not found in directory /lib/modules/5.7.0.0.bpo.2-amd64
+```
+
+and also this:
+
+```bash
+avra@vm-debian:~/can/can-utils$ testj1939
+testj1939: socket(j1939): Protocol not supported
+```
+
+
+
+### Solution
+
+Above errors mean that **can-j1939** was not enabled in your kernel and you need to compile it manually. There are several ways to do it. Any Linux kernel since 5.4 has **can-j1939** module, but you will probably want to install fresher version, which leads to downloading kernel sources, enabling **can-j1939** module, recompiling kernel and installing it. I will be using Debian 10.5 x64 (buster testing) virtual machine.
+
+
+
+#### 1. Download kernel source ####
+
+We will download Debian patched kernel 5.8. First update your sources
+
+```
+avra@vm-debian:~$ sudo apt update
+```
+
+and then see if what Debian pathed kernel source versions are available
+
+```
+avra@vm-debian:~$ apt-cache search linux-source
+linux-source-4.19 - Linux kernel source for version 4.19 with Debian patches
+linux-source - Linux kernel source (meta-package)
+linux-source-5.4 - Linux kernel source for version 5.4 with Debian patches
+linux-source-5.5 - Linux kernel source for version 5.5 with Debian patches
+linux-source-5.6 - Linux kernel source for version 5.6 with Debian patches
+linux-source-5.7 - Linux kernel source for version 5.7 with Debian patches
+linux-source-5.8 - Linux kernel source for version 5.8 with Debian patches
+```
+
+If kernel 5.8 does not show in your linux-sources list (it shows in mine since I have already upgraded stock 4.19 kernel to backported 5.7), then you will need to add backports to your sources list. Here is how my **/etc/apt/sources.list** looks like (you will need to append at least last line to yours):
+
+```
+deb http://security.debian.org/debian-security buster/updates main contrib
+deb-src http://security.debian.org/debian-security buster/updates main contrib
+
+deb http://deb.debian.org/debian/ buster main contrib non-free
+deb-src http://deb.debian.org/debian/ buster main contrib non-free
+
+deb http://deb.debian.org/debian buster-backports main contrib non-free
+```
+
+After adding backports try **sudo apt update** again, and **apt-cache search linux-source** should now show kernel 5.8 in the list, so you can install it's source package
+
+```
+sudo apt install linux-source-5.8
+```
+
+and unpack it
+```
+avra@vm-debian:/usr/src$ cd /usr/src
+avra@vm-debian:/usr/src$ sudo tar -xaf linux-source-5.8.tar.xz
+avra@vm-debian:/usr/src$ cd linux-source-5.8
+```
+
+
+
+#### 2. Add can-j1939 module to kernel ####
+
+First we need some packages for **menuconfig** 
+
+```
+sudo apt-get install libncurses5 libncurses5-dev
+```
+
+copy and use our old configuration to run **menuconfig**
+
+```
+avra@vm-debian:/usr/src/linux-source-5.8$ sudo cp /boot/config-$(uname -r) .config
+avra@vm-debian:/usr/src/linux-source-5.8$ sudo make menuconfig
+```
+
+where we enable SAE  J1939 kernel module as shown
+
+```
+	- Networking Support
+		- Can bus subsystem support
+			- <M> SAE J1939
+```
+
+Now edit **/usr/src/linux-source-5.8/.config**, find below key and change it as this
+```
+CONFIG_SYSTEM_TRUSTED_KEYS=""
+```
+
+and save it.
+
+
+
+#### 3. Compile and install kernel and modules
+
+We will have to download necessary packages
+
+```
+sudo apt install build-essential libssl-dev libelf-dev bison flex
+```
+
+compile kernel (using threads to make it faster)
+
+```
+avra@vm-debian:/usr/src/linux-source-5.8$ sudo make -j $(nproc)
+```
+
+install
+
+```
+avra@vm-debian:/usr/src/linux-source-5.8$ sudo make modules_install
+avra@vm-debian:/usr/src/linux-source-5.8$ sudo make install
+```
+
+and update grub
+
+```
+avra@vm-debian:/usr/src/linux-source-5.8$ sudo update-grub
+avra@vm-debian:/usr/src/linux-source-5.8$ sudo reboot
+```
+
+Check if installation is correct with
+
+```
+sudo modprobe can-j1939
+```
+
+and if you get no error then you can enjoy **can-j1939**. If you get some error then you might check if this alternative command works:
+
+```
+sudo insmod /lib/modules/5.8.10/kernel/net/can/j1939/can-j1939.ko
+```
+
+If it does then all you need to do is 
+
+```
+sudo depmod -av
+```
+
+reboot once, and **modprobe** command from the above should finally work now.
+
+
+
+#### 4. Install headers if needed
+
+You can have a problem with header file, to check that go in the file **/usr/include/linux/can.h**
+
+```
+nano /usr/include/linux/can.h
+```
+
+If in the struct **sockaddr_can** you don’t see **j1939**, then header files did not upgrade, so you need to do this manually
+
+```
+sudo cp /usr/src/linux-source-5.8/include/uapi/linux/can.h /usr/include/linux/can.h
+sudo cp /usr/src/linux-source-5.8/include/uapi/linux/can/j1939.h /usr/include/linux/can/
+```
+
+This is the minimum for compiling some **J1939** code,  but you might want to upgrade other header files as well. That's up to you. Enjoy!