Another thing:

> The next step is to run
>
> ```shell
> make localmodconfig
> ```
>
> and note the output.  Do note that the '.config' file is still empty.
> The output generally contains two types of warnings.  The first start
> with "WARNING" and can actually be ignored in our case.  The second read:
> ```shell
> module pcspkr did not have configs CONFIG_INPUT_PCSPKR
> ```
>
> For each of these lines, copy the CONFIG_XXXX_XXXX portion into the
> '.config' in the directory, and append "=m", so in the end it looks
> like this:
> ```shell
> CONFIG_INPUT_PCSPKR=m
> CONFIG_VIRTIO=m

Since I'm not starting from a blank .config, I wanted to compare the
output of `make localmodconfig' and, even better, automatically enable all
the missing modules.

In my case there was some 150 modules and that would have been a lot of work.

So I wrote those quick and dirty Emacs functions:

--8<---------------cut here---------------start------------->8---
(defun kernel-compare-configs (file1 file2)
  "Go through all CONFIG_* variables in FILE1 and report those in
  file2 which are not set."
  (interactive "f\nf")
  (let ((buffer1 (find-file-noselect file1))
        (buffer2 (find-file-noselect file2))
        result)
    (with-current-buffer buffer1
      (goto-char (point-min))
      (while (search-forward "CONFIG_" nil 'noerror)
        (let ((var (symbol-name (sexp-at-point))))
          (with-current-buffer buffer2
            (goto-char (point-min))
            (when (search-forward var nil 'noerror)
              (forward-char)
              (let ((value (word-at-point)))
                (unless (or (string= value "m")
                            (string= value "y"))
                  (push var result))))))))
    result))

(defun kernel-enable-configs (file variables)
  "Variables must be a list of strings.
Example:
  (\"CONFIG_MODULES\")"
  (let ((buffer (find-file-noselect file)))
    (with-current-buffer buffer
      (dolist (var variables)
        (goto-char (point-min))
        ;; Match exact variable only, e.g. CONFIG_MODULES should not match
        ;; CONFIG_MODULES_TREE_LOOKUP.
        (if (not (re-search-forward (concat var "[= ]") nil 'noerror))
            (warn "Variable %S not found" var)
          (beginning-of-line)
          (let ((kill-whole-line nil))
            (kill-line))
          (insert (format "%s=y" var)))))))

;; Example:
; (kernel-enable-configs
;  "/home/ambrevar/.guix-packages/ambrevar/linux-laptop.conf"
;  (compare-kernel-configs
;   "localmodconfig.config"
;   "/home/ambrevar/.guix-packages/ambrevar/linux-laptop.conf"))
--8<---------------cut here---------------end--------------->8---

The above is not general enough but that's a start.

I wonder if there aren't some dedicated tools out there already.  Anyone?

-- 
Pierre Neidhardt
https://ambrevar.xyz/