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power_supply.txt
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A seemingly simple question "is this computer running on mains power" turns out to be really complex to answer. Here's a simplified overview. An electrical connection doesn't imply a power sensor, for this you need appropriate circuitry. Thus, most desktops and servers don't have any sense of where their power comes from. Here, assuming this means AC is quite reasonable -- but on_ac_power doesn't make this decision for you, so you know you should avoid displaying an "AC plug" icon, etc. A typical laptop has one mains plug and one battery. As running out of battery causes data loss, all laptops have appropriate sensors (but such sensors might still lack supported drivers). High-end servers have multiple mains connectors. Some laptops have multiple batteries. Phones and tablets may have different charging modes ("trickle", normal, fast). But in some cases, all hell goes loose. For example, Allwinner A64 SoC knows about three sources: AC, USB, battery, yet particular boards using it don't match those. Pine64 for one has "ac" on an USB type B plug, a "battery" that watches a battery attachment, USB OTG type A that can power the board yet lacks PMIC circuitry to signal that it's connected (thus "usb" will always return online=0), and can be also powered via GPIO pins on Euler and Exp connectors, which don't tell the kernel anything at all. At this time, mainline kernel lacking proper drivers to tell us this doesn't feel that bad... Available interfaces: Modern ====== In /sys/class/power_supply/ you get one subdirectory per power source, with fairly straightforward data. The main thing to look at is "online" which tells us whether that source is connected. Type "type" can be one of: "Unknown", "Battery", "UPS", "Mains", "USB", "USB_DCP", "USB_CDP", "USB_ACA", "USB_C", "USB_PD", "USB_PD_DRP", "BrickID" "Mains" can also mean any other known but non-specific connector. It seems that usually USB describes itself this way. The difference between those many USB* variants seems to be irrelevant for our purposes. I have yet to see any that would provide any data about where the power actually comes from. Thus, we can't tell if we're on an USB charger vs a powerbank. But that's no different from AC coming from a disconnected UPS... "BrickID" is an Apple specific charger that I don't know anything about. The vast majority of UPSes don't show up as "UPS" which stands only for some specific type of GPIO control connector (which I don't know). Usually, we'd need to ask an userspace daemon like nut. "Wireless" is not reported by any driver in the mainline kernel, but is used by some out-of-tree devices. Legacy ====== ACPI: /proc/acpi/. Not interesting for us as all(?) drivers also provide a modern sysfs interface. APM: /proc/apm. Present only on some very old 32-bit x86 machines; the kernel issues real mode calls to the BIOS. In Debian, i386 kernels still ship APM modules, although to enable them you need to manually specify "acpi=off apm=on" on the kernel's command line. PMU: /proc/pmu/, specific to Apple Powerbooks. The driver does _not_ provide data in /sys/. Userspace ========= Most UPSes are not recognized by the kernel, and talk to an userspace program such as nut. on_ac_power doesn't currently query nut nor any other daemon of this kind. Note that the UPS provides its stored electricity as AC or DC that will appear to the kernel as mains power. Thus, the UPS' answer needs to override sysfs data.