Problem #2: How to add a wireless module to a d-i image?

Slightly longer version: As reported in #686605, a whole wireless module family is missing from the linux kernel udebs, meaning they’re missing from the d-i images. Please note the hypothesis here is that the regular linux kernel images contain the relevant modules, only the udebs don’t.

Recipe #2: Craft a local udeb!

1. The following assumes one is interested in adding a wireless module, but that can be adapted to other module families. The relevant udeb base name is nic-wireless-modules.
2. When it comes to linux kernel modules, picking the right ones is important, so one has to consider the ABI. Right now, the current ABI in testing is 3.2.0-3-amd64. The complete udeb name is therefore nic-wireless-modules-3.2.0-3-amd64-di.
3. If you’re running a kernel with the same ABI, all good, relevant modules should be available under /lib/modules/$(uname -r), which is what will be assumed below. Otherwise, one would have to download a proper linux-image-$ABI package, and extract it into a temporary directory to grab the wanted module(s).

4. Determine the relevant module and its dependencies. As an example, I’ll pick a Realtek module: rtl8192cu.ko. If it’s loaded, one can use the following to detect its dependencies:

   $ lsmod|grep rtl8192cu
   rtl8192cu              78863  0
   rtlwifi                81350  1 rtl8192cu
   rtl8192c_common        52602  1 rtl8192cu
   mac80211              192768  4 iwlwifi,rtl8192c_common,rtlwifi,rtl8192cu
   cfg80211              137140  3 mac80211,iwlwifi,rtlwifi
   usbcore               128498  7 ehci_hcd,usbhid,uvcvideo,rtlwifi,rtl8192cu
   

   But it isn’t even needed to load the module to get those dependencies:

   $ /sbin/modinfo /lib/modules/3.2.0-3-amd64/kernel/drivers/net/wireless/rtlwifi/rtl8192cu/rtl8192cu.ko|grep ^depends
   depends:        rtlwifi,mac80211,rtl8192c-common,usbcore
   

   Compared to what the d-i image contains, only rtlwifi and rtl8192c-common are missing.

5. Fetch the udeb using http://packages.debian.org/ or browsing your favourite mirror if you know the paths by heart.

6. Extract the udeb into a temporary data/ directory, along with its meta data:

   dpkg -x nic-wireless-modules-3.2.0-3-amd64-di_3.2.23-1_amd64.udeb data
   dpkg --control nic-wireless-modules-3.2.0-3-amd64-di_3.2.23-1_amd64.udeb data/DEBIAN
   

7. Copy the 3 modules under the relevant subdirectory under data/.

8. Update the version in data/DEBIAN/control, for example by adding the +local1 suffix.

9. Build a new binary package, using fakeroot to avoid owner/group noise in the diff:

   fakeroot dpkg -b data nic-wireless-modules-3.2.0-3-amd64-di_3.2.23-1+local1_amd64.udeb
   

10. Use debdiff to check the results:

    $ debdiff nic-wireless-modules-3.2.0-3-amd64-di_3.2.23-1*_amd64.udeb
    Files in second .deb but not in first
    -------------------------------------
    -rw-r--r--  root/root   /lib/modules/3.2.0-3-amd64/kernel/drivers/net/wireless/rtlwifi/rtl8192cu/rtl8192c-common.ko
    -rw-r--r--  root/root   /lib/modules/3.2.0-3-amd64/kernel/drivers/net/wireless/rtlwifi/rtl8192cu/rtl8192cu.ko
    -rw-r--r--  root/root   /lib/modules/3.2.0-3-amd64/kernel/drivers/net/wireless/rtlwifi/rtlwifi.ko
    
    Control files: lines which differ (wdiff format)
    ------------------------------------------------
    Version: [-3.2.23-1-] {+3.2.23-1+local1+}
    

11. Put the resulting udeb under localudebs/ in your d-i build tree, and rebuild your image as usual. Almost-tada!

Why “almost”? Because a firmware is needed, so one can go back to the 7th step, extract the proper file (lib/firmware/rtlwifi/rtl8192cufw.bin) from the non-free firmware-realtek package, and add it under data/lib/firmware/realtek/. Rebuild the udeb, rebuild the d-i image. Then: tada!