freefall is a simple daemon providing HDD shock protection for HP laptops supporting the feature officially called "HP Mobile Data Protection System 3D" or "HP 3D DriveGuard".
You need to activate the following kernel option either as built-in or as module.
Device drivers ---> [*] X86 Platform Specific Device Drivers ---> <*> HP laptop accelerometer
The freefall daemon and init script can be found in sys-apps/linux-misc-apps:
emerge --ask sys-apps/linux-misc-apps
Set the HDD in the /etc/conf.d/freefall configuration file.
Start freefall daemon:
To start freefall at boot time, add it to your boot runlevel:
rc-update add freefall boot
After reboot with new kernel, check if hp_accel driver was initialized correctly:
dmesg | grep hp_accel
Testing shock protection
Be careful while performing this test and keep the laptop above something soft. Simulating free fall over the desk or a hard floor is bad idea!
Find the HDD's unload_heads file, it will be somewhere in the /sys directory:
find /sys -name unload_heads
Go to its directory and run:
watch --difference=permanent cat unload_heads
Lift the laptop into free space and simulate free fall while holding it firmly. About a 10 cm drop should be enough.
If the disk protection works then the HDD should make "click" sound, see one of your laptop's LEDs flashing and the watch value's background should permanently turn black.