linux-kernel - Re: Generic battery interface

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Date:	Fri, 28 Jul 2006 03:27:00 +0300
From:	"Shem Multinymous" <multinymous@...il.com>
To:	"Vojtech Pavlik" <vojtech@...e.cz>
Cc:	"Brown, Len" <len.brown@...el.com>, "Pavel Machek" <pavel@...e.cz>,
	"Matthew Garrett" <mjg59@...f.ucam.org>,
	"kernel list" <linux-kernel@...r.kernel.org>,
	linux-thinkpad@...ux-thinkpad.org, linux-acpi@...r.kernel.org
Subject: Re: Generic battery interface

Hi,

On 7/28/06, Vojtech Pavlik <vojtech@...e.cz> wrote:
> > Battery polling is already used extensively, and its overhead is
> > completely negligible.
>
> You're joking, right? On quite a number of laptops, it takes quite a
> while to read the battery, spent in BIOS through SMI, polling the I2C
> bus while talking to the battery. The less often this is done, the
> better.

Yes, I know -- tp_smapi does that too. And it's still negligible,
usually a few microseconds.

Heck, the hdaps driver polls that same I2C bus 50 times per seconds
and still doesn't tickle the load average.

> > I'm yet to see any deployed userspace code
> > trying to query battery status more than once per second.
>
> The applets that were doing it (yes, up to 100 times per second)
> corrected their ways pretty quickly, because some machines became
> unusable with the applet enabled.

Exactly -- and they've been working merrily ever since.
And if you don't want to trust applet developers, cache the latest
reads and refresh them only if X jiffies have passed.

> You could, trivially, mirror the behavior of current applets: Not report
> the changes to the battery status more often than each N seconds, except
> for critical events.

You're taking a polling-based hardware, exposing it as an event-based
interface, and then and kludging it so that it behaves like polling
again...

So, in this scheme, how many lines of code does is the equivalent of
"cat /sys/devices/platform/smapi/BAT0/voltage"?

> > And you'll need to identify devices in a useful way, a problem that's
> > not yet solved even for input devices... You see where it's going.
>
> May you be more specific here? I'm not aware of any problems in this
> area. This may be my fault: What needs to be fixed there?

"Generic interface for accelerometers (AMS, HDAPS, ...)" on LKML, a
few weeks ago, about moving accelerator-based hard disk parking from
sysfs polling to the the input infrastructure. One unresolved issue
was how to find which input device happens to be the relevant
accelerometer.

  Shem
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