linux-kernel - Re: Battery class driver.

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Message-ID: <41840b750610232004k46e4937emfcdff4f1aef0b30f@mail.gmail.com>
Date:	Tue, 24 Oct 2006 05:04:17 +0200
From:	"Shem Multinymous" <multinymous@...il.com>
To:	"David Zeuthen" <davidz@...hat.com>
Cc:	"Greg KH" <greg@...ah.com>,
	"David Woodhouse" <dwmw2@...radead.org>,
	linux-kernel@...r.kernel.org, olpc-dev@...top.org,
	mjg59@...f.ucam.org, len.brown@...el.com, sfr@...b.auug.org.au,
	benh@...nel.crashing.org
Subject: Re: Battery class driver.

On 10/24/06, David Zeuthen <davidz@...hat.com> wrote:

>  b) if the kernel ensures that the 'timestamp' file is updated last,
>     we get atomic updates

Atomic updates require either double-buffering the data (twice worse
than mere caching...) or locking away access during update (in which
case the order doesn't mater).

But yes, lack of atomicity in sysfs is a big issue. We don't seem to
have any ABI convention for providing atomic snapshots of those
dozen-small-atribute directories.

>     Notably user space can see _when_ the values from the hardware was
>     retrieved the last time

> This is a good thing, but is orthogonal to the how-do-we-poll issue.

> So.. how all this relates to hwmon I'm not sure.. looking briefly at
> Documentation/hwmon/sysfs-interface no such thing seems to be available,

Yes, hwmon ignores merrily ignores this issue, as do all other
data-through-sysfs drivers I've looked at. Except for the patched
hdaps driver in the tp_smapi package, which has a (very) rudimentary
solution via caching and a configurable in-kernel polling timer.

  Shem
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