Querying battery information w/o envstat

grtcdr

Hey everybody!

I'm an author of a system information fetcher that takes performance quite seriously, and lately, we've started to cut down on the usage of external programs for fetching system data. NetBSD's implementation performance is currently running behind other supported platforms and I'd very much love to see it be on-par with all of them.

Do you know of any way to query battery statistics, such as battery percentage and battery capacity without the use of envstat?

Thanks 😀

rvp

grtcdr Assuming you don't want to link against the proplib library, you can just issue the ioctl, retrieve the XML dictionary plist, then parse the XML in whatever language you want to get battery stats.

The following program just dumps the XML retrieved from the kernel:

/**
 * Print the global property dictionary (XML) on NetBSD.
 */
#include <sys/types.h>
#include <sys/envsys.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <prop/plistref.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(void)
{
	struct plistref P;
	int fd, rc = EXIT_FAILURE;
	char* dev = "/dev/sysmon";

	if ((fd = open(dev, O_RDONLY)) == -1)
		err(rc, "%s: open failed.", dev);

	if (ioctl(fd, ENVSYS_GETDICTIONARY, &P) == -1)
		err(rc, "%s: ioctl failed.", dev);

	/*
	 * ripped from:
	 * src/common/lib/libprop/prop_kern.c
	 */
	if (P.pref_len == 0)
		errx(rc, "pref_len == 0");
	printf("%.*s", (int)P.pref_len, (char* )P.pref_plist);
	munmap(P.pref_plist, P.pref_len);
	close(fd);
	
	return 0;
}

nia

envstat uses envsys(4) ioctls

grtcdr

nia

Hi!
Thank you, I'll take a look at it!

grtcdr

rvp

Hey there, and thanks for taking the time 🙂

I'll be using this as a reference along the way. I'm not familiar with proplib, but it looks like it provides a similar interface to sysctl where I can obtain the value of a property without having to parse it myself, is this the case?

rvp

grtcdr Yes, proplib will parse the XML, and, also do the ioctl for you. Use the envstat code as a reference.

rvp

grtcdr There are few things you should keep in mind here:

A system can have more than one battery.
Individual batteries can report their SoC either in Watt-Hours or Ampere-Hours.
So you'll have to be prepared to convert to a common unit.
You can't rely on any battery sensor having a fixed description string like "charge" because this can be modified. (Either programmatically--the envsys(4) man-page has an example, or, using envstat -c conf_file.)
For example, I can change "charge" to "チャージ"--which is the same thing in Japanese--and envstat will display the new description.
So, to determine the current SoC you'll have to follow a procedure like this (all this is not specifically documented anywhere; one has to read between the lines a bit)--note that this only is for the acpibat and MacPPC smartbat battery drivers:

a) Walk the property list to find devices with a device-class of battery.

b) For each battery device, find the sensor with a want-percentage key.
Additionally, you can also check for warning-capacity and/or critical-capacity keys.
These will tell you if the sensor is a SoC sensor.

c) If such a sensor is found, check if its state is valid.

d) For all valid SoC sensors, read the current SoC from cur-value.
Convert to a common unit, if needed, based on the type key.

e) To determine the SoC %, you'll also need the max-value read (thankfully!) from the same sensor.