Chapter 7 Kernel Configuration

7.1. I would like to customize my kernel. Is it difficult?
7.2. My kernel compiles fail because _hw_float is missing. How do I solve this problem?
7.3. Why is my kernel so big (over 10MB)?
7.4. Why do I get interrupt conflicts with multi-port serial code?
7.5. Why does every kernel I try to build fail to compile, even GENERIC?

7.1. I would like to customize my kernel. Is it difficult?

Not at all! Check out the kernel config section of the Handbook.

Note: It is recommended that you make a dated snapshot of your kernel in kernel.YYMMDD after you get it all working, that way if you do something dire the next time you play with your configuration you can boot that kernel instead of having to go all the way back to kernel.GENERIC. This is particularly important if you are now booting off a controller that is not supported in the GENERIC kernel.

7.2. My kernel compiles fail because _hw_float is missing. How do I solve this problem?

Let me guess. You removed npx0 (see npx(4)) from your kernel configuration file because you do not have a math co-processor, right? Wrong! :-) The npx0 is MANDATORY. Even if you do not have a mathematic co-processor, you must include the npx0 device.

7.3. Why is my kernel so big (over 10MB)?

Chances are, you compiled your kernel in debug mode. Kernels built in debug mode contain many symbols that are used for debugging, thus greatly increasing the size of the kernel. Note that if you running a FreeBSD 3.0 or later system, there will be little or no performance decrease from running a debug kernel, and it is useful to keep one around in case of a system panic.

However, if you are running low on disk space, or you simply do not want to run a debug kernel, make sure that both of the following are true:

  • You do not have a line in your kernel configuration file that reads:

        makeoptions DEBUG=-g
    
  • You are not running config(8) with the -g option.

Both of the above situations will cause your kernel to be built in debug mode. As long as you make sure you follow the steps above, you can build your kernel normally, and you should notice a fairly large size decrease; most kernels tend to be around 1.5MB to 2MB.

7.4. Why do I get interrupt conflicts with multi-port serial code?

When I compile a kernel with multi-port serial code, it tells me that only the first port is probed and the rest skipped due to interrupt conflicts. How do I fix this?

The problem here is that FreeBSD has code built-in to keep the kernel from getting trashed due to hardware or software conflicts. The way to fix this is to leave out the IRQ settings on all but one port. Here is a example:

    #
    # Multiport high-speed serial line - 16550 UARTS
    #
    device sio2 at isa? port 0x2a0 tty irq 5 flags 0x501 vector siointr
    device sio3 at isa? port 0x2a8 tty flags 0x501 vector siointr
    device sio4 at isa? port 0x2b0 tty flags 0x501 vector siointr
    device sio5 at isa? port 0x2b8 tty flags 0x501 vector siointr

7.5. Why does every kernel I try to build fail to compile, even GENERIC?

There are a number of possible causes for this problem. They are, in no particular order:

  • You are not using the new make buildkernel and make installkernel targets, and your source tree is different from the one used to build the currently running system (e.g., you are compiling 4.3-RELEASE on a 4.0-RELEASE system). If you are attempting an upgrade, please read the /usr/src/UPDATING file, paying particular attention to the ``COMMON ITEMS'' section at the end.

  • You are using the new make buildkernel and make installkernel targets, but you failed to assert the completion of the make buildworld target. The make buildkernel target relies on files generated by the make buildworld target to complete its job correctly.

  • Even if you are trying to build FreeBSD-STABLE, it is possible that you fetched the source tree at a time when it was either being modified, or broken for other reasons; only releases are absolutely guaranteed to be buildable, although FreeBSD-STABLE builds fine the majority of the time. If you have not already done so, try re-fetching the source tree and see if the problem goes away. Try using a different server in case the one you are using is having problems.

This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.

For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.