Credit: Stephen Sauer
NEW ORLEANS - Linux kernel developer Sarah Sharp is searching for a solution to the problem posed by vampire mice, she said in a presentation at LinuxCon North America Wednesday afternoon.
“I don’t mean the little fuzzy, furry ones,” Sharp said. “I mean the ones that suck the life out of your battery.”
It’s a long-standing problem with USB power management in Linux environments. A lack of device driver support for Linux means that the power-saving features used in many USB devices – your mouse’s ability to essentially turn itself off if you haven’t moved it lately, for instance – are rendered ineffective or even made potentially damaging in some cases.
One such issue affected a particular brand of portable USB hard drive. Sharp said that an alarming problem occurred when the drive attempted to come out of suspend mode.
“The user hears this horrible scraping noise because the driver forgot to park the disc head before it cut power to the disk,” she said.
While this type of damage seems to be an outlier, much more common is the power-leeching issue that occurs when a device’s important suspend feature doesn’t work properly.
“If you have just one USB device that’s attached to the host that’s not suspended, that means that … your host’s hardware is continually touching the [USB] bus schedule,” she said.
Essentially, this means that even a single device – be it mouse, hard drive, or anything else – refusing to suspend properly means that the entire USB host must remain active, creating a small but measurable power drain.
“It’s about four watts, if you just have one USB device that’s not suspended,” said Sharp.
Four watts might not sound like much, but it could have a meaningful impact on the battery life of a laptop running unplugged – and spread systemically over a large set of computers in an office environment, run up business utility bills.
The kernel developers have had to essentially implement a whitelist policy, Sharp said – meaning that, to avoid potential malfunctions, the suspend feature on USB devices is deactivated by default, with exceptions for devices that are known to work properly and the ability to activate it from the Linux user-space.
“We really need a way for vendors and distros and users to trust their USB devices, figure out what’s broken, and what actually works,” she said.
Until that happens, then, watch out for the vampire mice.
Email Jon Gold at firstname.lastname@example.org and follow him on Twitter at @NWWJonGold.