LinuxKPI: add KERNEL_VERSION() macro

[bz]

Summary

Add a KERNEL_VERSION() macro which normally would be in linux/version.h.
On Linux that file is auto-generated and we are supporting more than
one Linux version in LinuxKPI anyway so any further defines in there
would likely be wrong.

Adding the macro helps to support (vendor/out of tree) drivers more
easily which are supporting multiple Linux versions.

MFC after: 3 days

[dumbbell]

What are the use cases for this macro?

I’m curious because of the relationship it could have with LINUXKPI_VERSION we define and compare in several places.

[bz]

In D57590#1319905, @dumbbell wrote:

What are the use cases for this macro?

I’m curious because of the relationship it could have with LINUXKPI_VERSION we define and compare in several places.

LINUXKPI_VERSION we define "readable" as 70000; that's how I found it, that's how I kept doing it.

The equivalent would be LINUX_VERSION_CODE which uses the KERNEL_VERSION encoding.
So for an external driver which checks against KERNEL_VERSION() I have to define:

CFLAGS+=        -DLINUX_VERSION_CODE=458752     # 7.0.0

Don't ask me; that's the way it seems to be.

[bz]

I should add; obviously if I define it as 0x70000 that would likely work too but seems less common (at least in the code I was looking at).

[bz]

In D57590#1319911, @bz wrote:

I should add; obviously if I define it as 0x70000 that would likely work too but seems less common (at least in the code I was looking at).

So if I think it to the end, in theory LINUXKPI_VERSION should have been 0xNNNNN and not just NNNNN and we would almost be good.

[dumbbell]

Thanks for the explanation!