linux-kernel - Re: Event notification when sysfs attribute changes

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Date:	Wed, 12 Aug 2009 16:32:48 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	"Rick L. Vinyard, Jr." <rvinyard@...nmsu.edu>
Cc:	Linux USB <linux-usb@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Event notification when sysfs attribute changes

On Wed, Aug 12, 2009 at 16:26, Rick L. Vinyard, Jr.<rvinyard@...nmsu.edu> wrote:
> I'd like to allow userspace applications to track and display state
> changes (without polling) in a driver that exports attributes via sysfs.
>
> What is the best way to notify userspace applications when a sysfs
> exported attribute changes?
>
> In particular I wanted to drive the notification in the *_store() function
> of the sysfs attribute.
>
> Also, ideally, the mechanism would allow multiple userspace applications
> to hook into the notification.

There is nothing like this which works out-of-the-box. It needs kernel
support for every attribute that should be able to do that. Then
poll() can block until the kernel signifies a change, and return with
ERR, so the listener wakes up and can read the attribute again.

Kay
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