I'm trying to understand the differences between 21.02 and 19.07.
One thing is very interesting to me. At 21.02, there is no hack/backport/pending for kernel 4.x at target/linux/generic.
So, does this mean 21.02 did not support kernel 4.x? why?
If we would like to upgrade device(with kernel 4.x) to 21.02, is it not suitable? why?
Please help. Thanks.
OpenWrt uses opensource drivers for all hardware.
Linux kernel used in OpenWrt has opensource drivers together with kernel source, it's not a separate thing you can install on top like with Windows.
So the only way to get more recent features and improvements of hardware drivers is to update the kernel, and drop support for older kernel since default images will not use the old kernel anymore.
For example WPA3 support for wifi is only available with modern 5.x and later kernels, I think on 4.x you would need to backport it, and it is a lot of work.
If you are limited to a specific kernel version because you have a vendor SDK with drivers that work only with that version, you can try adding again (copy-paste) the folders with the 4.x kernel from 19.07 and change the makefile in your device target to build the 4.x kernel again.
Then you can test and you probably need to also disable some wifi features that can be set from GUI like WPA3 or new things that were not in 19.07. Most other software should work.
Also if your device was using swconfig to configure the onboard switch in 4.x and now in latest OpenWrt is using the new DSA drivers, you probably want to change that in the target makefile too, so you get again the right Luci webpage for configuring the switch.
The best option if you want to use OpenWrt and update it for a few major releases (not just use an old firmware release forever) is to get a device that is supported by opensource drivers, so you don't need to care about kernel version changes. A lot of devices and devboards are supported.
When you upgrade, through one of the standard images for your device, you also upgrade to a newer kernel. OpenWRT is a complete Linux distribution targeted to routers and other network connectivity devices.
Thanks for explaining. This is exactly what I need. Have a good day. ^^
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