I found that there were lots of bugs filed on this problem -- kernel.org bug 12816, Ubuntu bug 414810, Fedora bug 519105 and so on. A few of them had terse comments like "Can you try this patched binary release?" but none of them had a source patch, or any hints as to the actual nature of the problem.
But one of them did point me to
/sys/class/backlight. In my working 220.127.116.11 kernel, that
directory contained a sony subdirectory containing useful files
that let me query or change the brightness:
echo 1 >/sys/class/backlight/sony/brightness
On my nonworking 18.104.22.168 kernel, I had /sys/class/backlight but there was no sony subdirectory there.
grep SONY .config in
the two kernels revealed that my working kernel had SONY_LAPTOP set,
while the nonworking one did not. No problem! Just figure out where
SONY_LAPTOP is in the configuration (it turns out to be at the very
end of "device drivers" under "X86 Platform Specific Device Drivers"),
make menuconfig, set SONY_LAPTOP and rebuild ... right?
make menuconfig showed me lots of laptop
manufacturers in the "Platform Specific" category, but Sony
wasn't one of them. Of course, since it didn't show the option it
also didn't offer a chance to read the help for the option either,
which might have told me what its dependencies were.
Time for a recursive grep of kernel source:
grep -r SONY_LAPTOP .
arch/x86/configs/i386_defconfig told me the option did indeed still exist, and where to find it. drivers/platform/x86/Kconfig lists the option itself, and says it depends on INPUT and RFKILL.
RFKILL? A bit more poking around located that one near the end of "Networking support", with the name "RF switch subsystem support". (I love how user-visible names for kernel options only marginally resemble the CONFIG names.) It's apparently intended for "control over RF switches found on many WiFi and Bluetooth cards," something I don't seem to need on this laptop (my WiFi works fine without it) -- except that the kernel for some reason won't let me build the ACPI brightness control without it.
So that's the secret. Go to "Networking support", set "RF switch subsystem support", then back up to "Device drivers", scroll down to the end to find "X86 Platform Specific Device Drivers" and inside it you'll now see "Sony Laptop Extras". Enable that, and the Fn-F5/F6 keys (as well as /sys/class/backlight/sony/brightness) will work again.
[ 10:07 Nov 02, 2009 More linux/kernel | permalink to this entry | ]